Supported Client Authentication Methods.

• client_secret_basic (default) uses the HTTP Basic authentication scheme to send Client.client_id | client_id and Client.client_secret | client_secret in an Authorization HTTP Header.
• client_secret_post uses the HTTP request body to send Client.client_id | client_id and Client.client_secret | client_secret as application/x-www-form-urlencoded body parameters.
• private_key_jwt uses the HTTP request body to send Client.client_id | client_id, client_assertion_type, and client_assertion as application/x-www-form-urlencoded body parameters. The client_assertion is signed using a private key supplied as an AuthenticatedRequestOptions.clientPrivateKey | options parameter.
• none (public client) uses the HTTP request body to send only Client.client_id | client_id as application/x-www-form-urlencoded body parameter.