Using openid NotSupportedAuthMethodError, if first in configuration is not supported

[bvolkmer]

When using OpenID, if .well-known/openid-configuration -> token_endpoint_auth_methods_supported has an unsupported authentication method in the first place, the BaseOAuth2.__init__ raises an error httpx_oauth.oauth2.NotSupportedAuthMethodError: Auth method private_key_jwt is not supported.

This is due to OpenID.__init__ taking the first, without checking, if there are supported ones in the list.

[frankie567]

Good catch, that's indeed a problem. We should take care of looking for client_secret_basic or client_secret_post in the list before raising NotSupportedAuthMethodError.

PR welcome!