[Feature] Add support for Unified Host with experimental flag

[tanmay-db]

What changes are proposed in this pull request?

This PR adds support for unified host:

• Separates client type from host type determination, deprecating is_account_client and replacing it with host_type and client_type properties using new HostType and ClientType
  enums
• Adds an experimental flag to indicate if a host is unified: experimental_is_unified_host
• Adds a workspace_id attribute to Config, which is necessary for workspace clients that talk to unified hosts
• Adds get_unified_endpoints() function, which is used in the OIDC endpoint resolution logic to discover OAuth endpoints on unified hosts
• Adds header injection logic in ApiClient which adds an X-Databricks-Org-Id header to requests made by workspace clients on unified hosts
• Adds comprehensive test coverage including unit tests for host/config type detection, OIDC endpoint resolution, header injection, and integration tests

Similar to what is done for databricks/databricks-sdk-go#1307

How is this tested?

• Unit tests
• Manually integration tested

- All existing integration tests pass

Note: Integration test would be added in another PR once the test infra supports spog

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

Trigger:
go/deco-tests-run/sdk-py

Inputs:

• PR number: 1135
• Commit SHA: fa365402fc79141a3400ca85d144a9487fa0f30d

Checks will be approved automatically on success.

[tanmay-db]

Verified external-browser auth flow works, for existing and spog use cases, browser is opened and the further API calls are successful.