multi: migrate from gRPC to Connect RPC

[torkelrogstad]

Connect is a replacement for gRPC. It is effectively
a simplified version of gRPC. gRPC clients can still communicate with Connect
servers, without having to make any changes at all. One of the big benefits of
Connect is that it is possible to use a completely plain HTTP client as well
(for example just curl!).

All of these work fine:

$ curl -X POST -H 'Content-Type: application/json' http://localhost:50051/cusf.mainchain.v1.WalletService/GetBalance
$ grpcurl -plaintext localhost:50051/cusf.mainchain.v1.WalletService/GetBalance
$ buf curl http://localhost:50051/cusf.mainchain.v1.WalletService/GetBalance

Why bother?

The biggest benefit is that clients no longer need to bother
with gRPC at all. gRPC is notoriously hard and complex to deal with.
Generated code, hairy libraries, TLS, lots of strange concepts. People get
frustrated, and give up! With this, we could consider dropping our
home-rolled JSON-RPC server implementation of the validator and wallet
services.

Another change we make is to remove the cusf_sidechain_proto Git submodule in
favor of remote code generation through the Buf Schema Registry. Code generation
plugins are configured in buf.gen.yaml, and the generated code is checked into
Git. This blows up the diff via lots and lots of generated code (not a big deal,
IMO). The upside is that we can remove a few build dependencies, and that we
avoid the Git submodule in favor of what (I believe) is a simpler way of
upgrading the proto files. Note that we could still do build-time code
generation while using Connect, if we wanted to. Personal preference of mine is
on checked-in generated code: it is very nice to be able to actually read and
examine whatever code we're interacting with, instead of having to dig into the
target folder if something is not working as expected.

One downside here is that the Connect library does not (yet) support the
reflection API. This is something I assume they'll be adding shortly. The
library is very rapidly iterated on, judging by GitHub activity.

This supersedes #333, as we no longer have to deal with local proto files at all.

TODO:

• look into how we can fix the missing health checks implemented via Add new connectrpc-health crate anthropics/connect-rust#128
• should we rename the serve_grpc_port and serve_grpc_addr parameters?