Support network transaction submission in RPC

[Mirko-von-Leipzig]

This is currently hard denied but we want to submit network transactions internally via the RPC gRPC service.

Let's add an optional node config item that allows this via a header key, which the ntx builder will set. The latter therefore also requires a config item to set the auth key.

[bobbinth]

We want to do this for v0.15 release, right? (optimistically, added it to the milestone)