Added with_product(...) and with_user_agent_extra(...) public functions to improve telemetry for mid-stream libraries

[nfx]

Changes

This PR solves three tracking challenges:

1. Adds consistent mechanism with Go SDK Public API. See https://github.com/databricks/databricks-sdk-go/blob/00b1d09b24aa9fb971bcf23f3db3e80bf2bec6fe/useragent/user_agent.go#L20-L31 and https://github.com/databricks/databricks-sdk-go/blob/00b1d09b24aa9fb971bcf23f3db3e80bf2bec6fe/useragent/user_agent.go#L49-L54

2. Some of our products, like UCX and Remorph, are used not only as standalone CLI, but also as mid-stream libraries in other products, where developers don't specify product name and product version within the WorkspaceClient. This results in missing tracking information for those integrations.

3. Mid-stream libraries, like blueprint, pytester, and lsql, do have their own versions, but they don't create manage sdk.WorkspaceClient and/or core.Config themselves, so we currently lack traffic attribution for those libraries. Technically, Databricks Connect falls into the same use-case.

Tests

Moved unit tests that are relevant to User-Agent verification from test_core.py to test_config.py to bring back consistency.

[github-actions]

This PR breaks backwards compatibility for databrickslabs/ucx downstream. See build logs for more details.

_{Running from downstreams #185}

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 57.71%. Comparing base (f50af60) to head (4624127).

Files	Patch %	Lines
databricks/sdk/config.py	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
+ Coverage   57.69%   57.71%   +0.02%     
==========================================
  Files          48       48              
  Lines       33079    33102      +23     
==========================================
+ Hits        19084    19104      +20     
- Misses      13995    13998       +3     

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