Migrate auth plugins into the core repository

[fregataa]

Move the TOTP, OpenID, and keypair authentication plugins from their separate plugin repositories into the backend.ai core repository (under src/ai/backend/manager/plugin/).

Motivation:

• These auth plugins tightly depend on core internal modules (models, API types, config provider) and break frequently when core refactors import paths (e.g., BA-5016, BA-5017).

• Keeping them in-tree allows them to be updated atomically with core changes, eliminating version drift.

• Both plugins are maintained by the same team and have no external distribution need.

Scope:

• TOTP plugin (backend.ai-totp-plugin) — BA-5581

• OpenID plugin (backend.ai-openid-plugin) — BA-5580

• Keypair auth plugin (backend.ai-auth-keypair-plugin) — BA-5584

JIRA Issue: BA-5579