Allow specification of custom client authenticator types using keycloak_openid_client resource

[JackHartley]

The keycloak_openid_client resource validates its client_authenticator_type property against a fixed list of in-built client authenticator types (client-secret, client-jwt, client-x509, and client-secret-jwt). However, developers can create custom client authenticators (see Keycloak Server Developer Guide) which would have developer-specified types.

Would it be okay to remove the validation applied to client_authenticator_type so that custom client authenticators can be specified?