go/common/crypto/signature/signers: Sort out the RPC situation

We now have 2 flavors of RPC backed signers:

• signers/plugin - A simplified net/rpc interface for use with go-plugin
• signers/remote - A more fully fleshed out gRPC interface for the remote-signer API.

We could consider unifying these as go-plugin supports plugins that use gRPC as the transport (though I do not know how nicely it plays with our custom codec situation).

The rationale for using net/rpc for the plugin signer was purely one of expediency, and I have no strong attachment to the interface, beyond "it works, so why mess with it". As the plugin package supports versioning I expect the transition to be fairly painless.

One side benefit of transitioning the plugin signer to gRPC would be the ability to write plugins in non-Go languages, but that also does not seem all that urgent.