Credential provider for OAuth jwt-bearer token exchange (e.g. Anthropic Workload Identity Federation) #115

Anthropic Workload Identity Federation exchanges an OIDC JWT at POST /v1/oauth/token (RFC 7523 jwt-bearer) for a short-lived Authorization: Bearer token. Actors have no JWT to exchange, and egress injection only supports static Secret values or a raw Actor JWT. The only option today is a long-lived API key.

Proposal

A token-exchange credential provider on credprovider.v1:

• URI ate-secret://token-exchange/<namespace>/<name> names an exchange: token URL, extra form params (federation_rule_id, organization_id, service_account_id, …), and audience. It uses the same atespace→namespace grant as k8s.io.
• The assertion is the provider's ServiceAccount token, from a fresh TokenRequest for every exchange (Anthropic rejects a reused jti).
• Access tokens are cached and shared between concurrent fetches. A token is handed out only while it has comfortably more time left than the gateway's 5-minute credential cache, as in Mint Google access tokens in the Kubernetes credential provider #47.
• Chart wiring through credentialProvider.additionalProviders (feat(helm): further credential providers for the egress gateway #90).

Questions

• Should the default identity be the provider's ServiceAccount, or the calling Actor's JWT (MintActorJwt)? The latter lets the authorization server tell Actors apart. authz_util_test.go already lists a token-exchange identity with the egress-gateway role.
• Should this provider live in this repo, or ship as a separate image? Mint Google access tokens in the Kubernetes credential provider #47 was closed without discussion.

kagent side: kagent-dev/kagent#3470. Happy to send the PR.