Support `private_key_jwt` client authentication for custom OIDC providers

Problem

Custom OIDC providers (POST /admin/custom-providers) can only authenticate to the provider's token endpoint with a client_secret (client_secret_basic / client_secret_post). There is no way to use private_key_jwt (RFC 7523, OIDC Core §9).

Many regulated IdPs only accept asymmetric client authentication: healthcare and government identity providers, and anything following FAPI 2.0, which does not allow shared secrets. For those IdPs, the custom provider feature cannot be used at all.

Reproduction

Tested against supabase/gotrue:v2.196.0, with a mock OIDC provider whose token endpoint only accepts private_key_jwt:

1. The provider registers fine. The discovery document advertises token_endpoint_auth_methods_supported: ["private_key_jwt"], but that field is not kept in the stored discovery_document, and the admin API has no parameter to set the auth method.
2. At /callback, GoTrue sends client_secret_basic, then retries with client_secret_post. It never sends client_assertion.
3. The login fails:

   oauth2: "invalid_client" "client authentication 'client_secret_post' not allowed; expected private_key_jwt"
   500: Unable to exchange external code
   

A reference client using private_key_jwt against the same mock gets an ID token, so the mock itself works. The only external-side code path is oauth2.Config.Exchange in internal/api/provider/custom_oauth.go, which only sends the client secret. master (v2.198.0-rc.21) has no client_assertion handling for external providers either.

Proposal

Add a token_endpoint_auth_method to custom providers (client_secret_basic | client_secret_post | private_key_jwt). For private_key_jwt:

• Accept a private signing key per provider (a PEM or JWK, stored like client_secret) plus an optional kid.
• At code exchange, send client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer and a client_assertion JWT: iss = sub = client_id, aud = token endpoint, short exp, unique jti.
• Optionally, publish the public key at a per-provider JWKS URL so IdPs that fetch client keys can register it.

This is additive. Existing providers keep the default client_secret_basic.

Workaround

Handle the OIDC login outside GoTrue, then create the GoTrue session through the admin API. That works, but it gives up the custom provider feature (claims allowlist, identity linking) for exactly the IdPs where it would help most.

The mock provider and a compose file that reproduces this are available if useful.