feat(auth_proxy): migrate auth_proxy lib

[natalian98]

This PR migrates the auth_proxy interface library from charmhub into charmlibs.

Library being migrated

• Charmhub-hosted library:
  • LIBAPI: 0
  • LIBPATCH: 4
  • LIBID: 83df2500c289431aab1567ac1b780926
• New charmlibs package: charmlibs.interfaces.auth_proxy

This is an interface library (charmlibs.interfaces.<name>, just init --interface).

Migration status

This is a bug compatible migration of the Charmhub-hosted library, releasing as version 1.0.0.

Package:

• Package initialised with just init or just init --interface.
• Code migrated to src/charmlibs/<name>/_<name>.py.
• Public API exported from __init__.py with __all__.
• Charmhub lib docstring moved to __init__.py (this is rendered in the docs).
• Charmhub-hosted LIBAPI and LIBPATCH version documented in the migrated module's docstring.
• LIBID, LIBAPI, LIBPATCH removed (or retained with a note on why).
• PYDEPS moved to pyproject.toml dependencies with appropriate constraints.
• Library version set for release (typically 1.0.0).

Repository metadata:

• .docs/reference/libs.yaml updated with entries for new and old libs.
• CODEOWNERS updated with a /<package>/ entry for the owning team.

Tests and docs:

• Unit tests migrated, plus functional and integration tests as appropriate.
• Unnecessary files created by just init have been removed (including test_version.py and tests/functional and tests/integration if unused).
• Diataxis docs (if any) migrated to <package>/docs/.

Interface library specific items

• Directory name exactly matches the interface name as written in charmcraft.yaml.
• Interface metadata added (or updated if necessary), or an issue created and tracked to do this as a follow-up task.
• Testing package added under interfaces/<name>/testing/ exporting relation_for_provider and relation_for_requirer if needed (see how-to guide), or an issue created and tracked to do this as a follow-up task.

Commit strategy

To make review easier, this PR is organized into focused, semantic commits:

• feat: migrate auth_proxy interface: Unified migration of the upstream library. Contains the core scaffolding, verbatim baseline files (library source, tests, and documentation), updated imports, and linter-enforced adjustments in a single coherent step.
• chore: update CODEOWNERS: Registry of monorepo code ownership for the auth_proxy interface directory.
• chore: update auth_proxy lib metadata: Registering the package in the monorepo's global configuration indices (interfaces/index.json and .docs/reference/libs.yaml).