In this commit we deprecate the sighash field from the
MuSig2SignSweepReq request in favour of using a psbt serialized sweep
transaction instead. This way the sever gains full transparency about
client sweep requests and can assemble the sighash to sign on its own.
To make it possible to replace the swapserverrpc module in the server,
we declare it as its own submodule. This allows the server to make
changes to its local protos (and replace loop's copy with the local
version), to use changes that are not yet merged to the client repo.
Protobuf does not allow naming conflicts for files within the same
process, because all proto messages register themselves in a global
registry.
This is problematic because the server's itests import the client's
looprpc package to make rpc queries to the loopd client, thus importing
duplicate common.proto and server.proto from the client's looprc package
(since they're both in there as well).
This change moves the server's proto files into their own directory so
that they are not imported when we want to use the client's files. We
cannot change the package name for the server, because that would be
a breaking change (the package name is included in URIS). Fortunately,
we have the go_package option which allows us to place generated files
in a different location.
