refactor: reuse gRPC clients

[juan518munoz]

Add ValidatorClient, a clonable thin wrapper around the gRPC client. This allows sharing of grpc channels accross actors.

Closes #1804

[Mirko-von-Leipzig]

Thank you! Could you add a changelog entry, e.g.

### Fixes

- Network transaction actors now share gRPC clients, reducing the number of file descriptors used

Could you confirm

Confirm that cloning a tonic gRPC client shares the underlying gRPC channel i.e. only consumes a single socket/file descriptor.

from #1804