Enable entityless featureviews

[codyjlin]

Signed-off-by: Cody Lin codyl@twitter.com

What this PR does / why we need it:
This PR executes on the Entityless FeatureViews RFC, with minor adjustments to Implementation 1:

1. Registry
   1. Create __entityless entity on every apply
   2. Attach it to any entityless FVs when they are initialized
   3. Hide it from registry_dump (namely, list_entities and list_featureviews)
2. Offline
   1. For each of the offline store providers, update the joins to be able to join with no entities (only the timestamp col)
   2. (I think this implementation detail differs from the RFC - I think it is much simpler than having to create the extra __entityless_id dummy column to join on a constant value, and then having to hide it later when selecting columns)
3. Materialization/Online
   1. Add __entityless_id column as empty string upon materialize (namely, pull_latest_from_table_or_query in each offline store provider)
   2. If get_online_features requests any features from an entityless FV, {"__entityless_id": ""} will be a join_key_row to be converted to a entity_row_proto before look up in the online store.

Which issue(s) this PR fixes:
Fixes #1737

Does this PR introduce a user-facing change?:

The user can now specify FeatureViews with an empty `entities` list.

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[codecov-commenter]

Codecov Report

Merging #1804 (e4bde7c) into master (b0f38ad) will increase coverage by 0.38%.
The diff coverage is 94.79%.

@@            Coverage Diff             @@
##           master    #1804      +/-   ##
==========================================
+ Coverage   84.13%   84.51%   +0.38%     
==========================================
  Files          90       90              
  Lines        6770     6827      +57     
==========================================
+ Hits         5696     5770      +74     
+ Misses       1074     1057      -17     

Flag	Coverage Δ
integrationtests	84.44% <94.79%> (+0.38%)	⬆️
unittests	63.82% <65.62%> (+0.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/feast/repo_operations.py	44.58% <0.00%> (+0.13%)	⬆️
...tegration/feature_repos/universal/feature_views.py	95.83% <80.00%> (-4.17%)	⬇️
sdk/python/feast/feature_store.py	93.14% <97.22%> (+0.17%)	⬆️
sdk/python/feast/driver_test_data.py	100.00% <100.00%> (ø)
sdk/python/feast/feature_view.py	86.57% <100.00%> (+0.27%)	⬆️
sdk/python/feast/infra/offline_stores/bigquery.py	82.06% <100.00%> (ø)
sdk/python/feast/infra/offline_stores/file.py	97.43% <100.00%> (+0.09%)	⬆️
sdk/python/feast/infra/offline_stores/redshift.py	90.99% <100.00%> (ø)
sdk/python/feast/infra/provider.py	88.98% <100.00%> (ø)
sdk/python/feast/online_response.py	95.00% <100.00%> (+0.12%)	⬆️
... and 14 more

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[codyjlin]

/kind feature

[achals]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: achals, codyjlin

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