Make token_endpoint_auth_signing_alg_values_supported optional in upstream OIDC providers metadata

[LeonWilzer]

token_endpoint_auth_signing_alg_values_supported is an optional value according to OIDC spec

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[LeonWilzer]

According to the OIDC spec token_endpoint_auth_signing_alg_values_supported is optional, but MAS demands the field in secure discovery mode, even if MAS isn't configured to use JWT. We are running into issues because of this.

[sandhose]

I think the problem is that I took the RFC8414 definition for this one:

token_endpoint_auth_signing_alg_values_supported
      OPTIONAL.  JSON array containing a list of the JWS signing
      algorithms ("alg" values) supported by the token endpoint for the
      signature on the [JWT](https://www.rfc-editor.org/rfc/rfc8414.html#ref-JWT) [JWT] used to authenticate the client at the
      token endpoint for the "private_key_jwt" and "client_secret_jwt"
      authentication methods.  This metadata entry MUST be present if
      either of these authentication methods are specified in the
      "token_endpoint_auth_methods_supported" entry.  No default
      algorithms are implied if this entry is omitted.  Servers SHOULD
      support "RS256".  The value "none" MUST NOT be used.

Thanks for this! I think I need to do a round and be less strict with many of those fields