feat(rest): implement OAuth2 token auto-refresh for REST catalog

[lishuxu]

Replace the MakeOAuth2 stub with a full OAuth2AuthSession that
automatically refreshes tokens before expiration using the
client_credentials grant.

Key components:
- OAuth2AuthSession: manages token lifecycle with shared_mutex for concurrent read access and background refresh via scheduler
- TokenRefreshScheduler: process-global singleton with a single worker thread that fires delayed refresh callbacks
- ExpiresAtMillis: JWT exp claim parser for determining token expiry when expires_in is not provided in the token response
- Base64Decode/Base64UrlDecode added to TransformUtil as public utilities