docs(samples): add retail search service samples

[t-karasova]

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 7 region tags.

• samples/interactive-tutorials/search/search_simple_query.py:15, tag retail_search_for_products_with_query_parameter
• samples/interactive-tutorials/search/search_with_boost_spec.py:15, tag retail_search_product_with_boost_spec
• samples/interactive-tutorials/search/search_with_facet_spec.py:15, tag retail_search_product_with_facet_spec
• samples/interactive-tutorials/search/search_with_filtering.py:15, tag retail_search_for_products_with_filter
• samples/interactive-tutorials/search/search_with_ordering.py:15, tag retail_search_for_products_with_ordering
• samples/interactive-tutorials/search/search_with_pagination.py:15, tag retail_search_for_products_with_page_size
• samples/interactive-tutorials/search/search_with_query_expansion_spec.py:15, tag retail_search_for_products_with_query_expansion_specification

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[t-karasova]

@kweinmeister , @chenlei1216
Hi Karl, Lei
I have created this pull request to add the code samples for Interactive Tutorials for Retail API.
You have already seen and approved them in #122, I am going to close that (122) as it is too big and the further review is difficult.

The code samples are moved to the folder "samples/interactive-tutorials" to separate them from the code samples, which are used for displaying in the static documentation pages.
It was decided, that Our tutorials + code samples will be shown only by clicking the button "Guide me" in the Retail documentation page and will be opened in the Cloud Shell IDE.

In this PR I have created the code samples for Retail Search service and tests.

I have modified .kokoro configs to read the secrets from "retail-samples-secrets" secret manager.

For the tests we use our own Cloud project with preset catalog data. And I would like to ask you how should I add the service account token to our project to authorize tests?

Thank you

[chenlei1216]

Maybe add code samples how to get the next page token?

[t-karasova]

We provide the code here, in the tutorial https://github.com/t-karasova/python-retail/blob/readme/samples/interactive-tutorials/tutorials/retail_api_v2_pagination_python.neos.md, and we ask user to copy it and paste under the specific comment in the code. we did it to increase the user engagement into the learning process.

[t-karasova]

the code is the following:

    search_request_next_page = get_search_request("Hoodie", page_size, offset, next_page_token)
    search_response_next_page = SearchServiceClient().search(search_request_next_page)
    print("---next page search results---")
    print(search_response_next_page)

[parthea]

Current failure for python 3.7 samples : pytest: error: unrecognized arguments: -n=auto

******************** TESTING PROJECTS ********************
------------------------------------------------------------
- testing samples/interactive-tutorials/search
------------------------------------------------------------
nox > Running session py-3.7
nox > Creating virtual environment (virtualenv) using python3.7 in .nox/py-3-7
nox > python -m pip install -r requirements.txt
nox > python -m pip install -r requirements-test.txt
nox > python -m pip install -e /workspace
nox > pytest --junitxml=sponge_log.xml
ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: -n=auto
  inifile: None
  rootdir: /workspace

nox > Command pytest --junitxml=sponge_log.xml failed with exit code 4
nox > Session py-3.7 failed.

 Testing failed: Nox returned a non-zero exit code. 

[parthea]

Added minor observations

[parthea]

The tests appear to be running now. There are 2 unit test failures:

=========================== short test summary info ============================
FAILED search_with_filtering_test.py::test_search_with_filtering - AssertionE...
FAILED search_with_query_expansion_spec_test.py::test_search_with_query_expansion_spec
========================= 2 failed, 12 passed in 6.14s =========================

=================================== FAILURES ===================================
__________________________ test_search_with_filtering __________________________

    def test_search_with_filtering():
        response = search()

        assert len(response.results) == 10
        product_title = response.results[0].product.title
>       assert re.match(".*Tee.*Black", product_title)
E       AssertionError: assert None
E        +  where None = <function match at 0x7f4bae37c710>('.*Tee.*Black', 'Google Black Cloud Tee')
E        +    where <function match at 0x7f4bae37c710> = re.match

search_with_filtering_test.py:39: AssertionError

____________________ test_search_with_query_expansion_spec _____________________

    def test_search_with_query_expansion_spec():
        response = search()

>       assert len(response.results) == 10
E       assert 1 == 10
E        +  where 1 = len([id: "GGOEGAYJ136014"\nproduct {\n  name: "projects/<REDACTED>/locations/global/catalogs/default_catalog/branches/0/p... 300\n    width: 400\n  }\n  color_info {\n    color_families: "Gray"\n    colors: "Light gray"\n    colors: "Silver"\n  }\n}\n])
E        +    where [id: "GGOEGAYJ136014"\nproduct {\n  name: "projects/<REDACTED>/locations/global/catalogs/default_catalog/branches/0/p... 300\n    width: 400\n  }\n  color_info {\n    color_families: "Gray"\n    colors: "Light gray"\n    colors: "Silver"\n  }\n}\n] = SearchPager<results {\n  id: "GGOEGAYJ136014"\n  product {\n    name: "projects/<REDACTED>/locations/global/catalogs/<REDACTED>"\n>.results

search_with_query_expansion_spec_test.py:39: AssertionError