What kind of change does this PR introduce?

This PR introduces a bugfix for developer ergonomics/import discovery.

It adds lazy-exported integration submodules in authlib.integrations.__init__ (including starlette_client) so IDEs/type checkers can resolve authlib.integrations.starlette_client correctly without forcing eager imports of optional integrations.

Related issue/PR: N/A.

Does this PR introduce a breaking change?

No.

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
• If this PR is about a new feature, or a behavior change, you have updated the documentation accordingly. (N/A)

• You consent that the copyright of your pull request source code belongs to Authlib's author.
  This pull request refactors the authlib/integrations/__init__.py file to implement lazy loading for its submodules. This approach optimizes import times and resource usage by only loading submodules when they are actually accessed.

Key changes:

Lazy Loading Implementation:

• Introduced a __getattr__ function that uses importlib.import_module to dynamically import submodules listed in __all__ when accessed as attributes. This enables lazy loading of integration submodules.

Type Checking Support:

• Added a TYPE_CHECKING block to ensure that type checkers are aware of the available submodules without triggering actual imports at runtime.

Module Export Definition:

• Defined an explicit __all__ list to specify the public submodules available for import from the authlib.integrations package.