Support partners in SDK

[mgyucht]

Changes

Ports databricks/databricks-sdk-go#925 to the Python SDK.

Partners of Databricks need a mechanism to register themselves in libraries or applications that they write. In this way, requests made by users of those libraries will include sufficient information to link those requests to the original users.

This PR adds a new function with_partner that can be used by a partner to add partner information to the User-Agent header for requests made by the SDK. The new header will have the form partner/<parther id>. The partner identifier is opaque for the SDK, but it must be alphanumeric.

This PR also removes the requirement that a user agent entry contain only a single copy of each key. This allows multiple partners to register in the same library or application.

In this PR, I've also refactored the user agent library to be more static, aligning it with the Go and Java SDKs. This makes it easier to maintain & ensures similar behavior between all 3 SDKs. Note that there is extra functionality in this SDK that doesn't exist in the Go and Java SDKs, namely config-level user agent info; that is preserved in here.

Tests

Unit tests were added to verify user agent contains all expected parts, and that the user agent supports multiple partners.

• make test run locally
• make fmt applied
• relevant integration tests applied

[github-actions]

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

_{Running from downstreams #128}

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.22034% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 57.69%. Comparing base (b13042b) to head (e0a8991).

Files	Patch %	Lines
databricks/sdk/user_agent.py	94.54%	3 Missing ⚠️
databricks/sdk/config.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #648      +/-   ##
==========================================
+ Coverage   57.66%   57.69%   +0.03%     
==========================================
  Files          48       49       +1     
  Lines       32680    32717      +37     
==========================================
+ Hits        18844    18877      +33     
- Misses      13836    13840       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[github-actions]

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

_{Running from downstreams #166}