[Client] Support token_endpoint_auth_method: client_secret_basic, client_secret_post, none #321

Context

Per RFC 6749 §2.3 (and AS metadata token_endpoint_auth_methods_supported), the client must send its credentials to the token endpoint using whichever method the AS advertises:

• client_secret_basic → HTTP Basic Authorization: Basic base64(client_id:client_secret)
• client_secret_post → form-encoded client_id + client_secret body params
• none → public client; only client_id in body, no secret

Scope

• Strategy interface TokenEndpointAuthInterface + three implementations.
• Auto-select implementation based on AS metadata's preferred method (intersect with what the client supports/has).

Conformance scenarios unblocked

auth/token-endpoint-auth-basic, auth/token-endpoint-auth-post, auth/token-endpoint-auth-none.

Dependencies

Blocked by: #318, #319.

Acceptance

• Unit tests per strategy.
• Conformance: 3 baseline scenarios pass.

cc @soyuka