Support `private_key_jwt` client authentication for custom OIDC providers
Axel Vanraes requests support for `private_key_jwt` client authentication in custom OIDC providers, as current options only allow `client_secret` methods. This is necessary for compliance with regulated identity providers that require asymmetric authentication. A proposal is made to add `token_endpoint_auth_method` support for `private_key_jwt`, including handling of JWT assertions. Warwick Hope supports this, highlighting its importance for Microsoft Entra ID users.