With go 1.17 a change to the build flags was implemented:
https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md
The formatter now automatically adds the forward-compatible build tag
format and the linter checks for them, so we need to include them in our
code.
We'll want to re-use the PSBT funding functionality from the wallet kit
sub server in the main RPC server. To be able to dynamically obtain an
instance of the wallet kit server we need to know its name under which
it registers itself in the list of sub server. We export the name so we
don't have to hard code it in the main server.
In this commit, we implement the newly defiend WalletKitServer gRPC
service. We use the same template w.r.t build tags as the existing
signrpc service.
