feat(rest): introduce AuthManager/AuthSession and migrate OAuth2

[plusplusjiajia]

Modeled on Java's AuthManager API (the init/catalog session lifecycle, the Noop/OAuth2 manager set, and the SigV4-wraps-a-delegate composition coming in the follow-up), adapted to Rust idioms.

What it does

• AuthManager/AuthSession traits in a new auth/ module: init_session() serves the GET /v1/config handshake, catalog_session(merged_props) serves everything after, so a manager can rebuild its session from server-merged properties.
• Noop/OAuth2 managers, selected via a new rest.auth.type property (oauth2 is the default and behaves as no auth when neither token nor credential is set), injectable through RestCatalogBuilder::with_auth_manager.
• OAuth2 token handling moves out of HttpClient into OAuth2Manager, with the cached token surviving the config handshake; OAuth2Manager is publicly constructible (new() + with_*).
• Review items from feat(catalog-rest): introduce AuthManager/AuthSession, rework SigV4 as a wrapping auth manager #2815 folded in: the config field is auth_manager, and the test-only fake-request token shim is gone — tests observe the session's cached bearer (#[cfg(test)] bearer_token()) and assert the header the mock server receives.

No new dependencies; no public API removed (additions only, public-api.txt regenerated).

Java reference: org.apache.iceberg.rest.auth.

Deviations from Java

• No tableSession/contextualSession yet — in Java they are default methods falling back to the catalog/parent session, and the Rust REST catalog has no call sites for them (contextualSession also needs a SessionCatalog concept that doesn't exist here yet). Adding defaulted trait methods later is non-breaking.
• No close() — Rust relies on Drop, and this OAuth2 implementation has no background refresh executor to shut down.
• AuthSession gains invalidate()/refresh() (not in Java) to back the existing RestCatalog::invalidate_token/regenerate_token APIs.
• authenticate mutates the request in place instead of returning a new one.

[plusplusjiajia]

Split from #2815 per your review, @CTTY — first of two: the AuthManager/AuthSession traits + OAuth2 migration (pure refactor, existing OAuth2 tests pass unmodified). SigV4 follows by reworking #2660 on top.
Would appreciate a review when you have a chance.