Extract credential interface from *defaultCredential to support both
default (OAuth) and external (remote proxy) credential types. External
credentials proxy requests to a remote ccm/ocm instance with bearer
token auth, poll a /status endpoint for utilization, and parse
aggregated rate limit headers from responses.
Add allow_external_usage user flag to control whether balancer/fallback
providers may select external credentials. Add status endpoint
(/ccm/v1/status, /ocm/v1/status) returning averaged utilization across
eligible credentials. Rewrite response rate limit headers for external
users with aggregated values.
a8934be7cd