Those endpoints use extensive amount of user-provided properties
and will be frequently used by third party developers. With Node
15, unhandled promise rejections directly crash the server, as
such, it is safer to schema validate the request before processing
it.
This change also prepares the code paths for later change that adds
destination fallbacks.
This simply won't work as we now support multiple clients. Each
client needs its own clientGatewayService which can only be
created by the user config.
Thus, let the form update user config directly.
