bug: RPCCall should also be generic just like QuorumCall

[meling]

This signature:

func RPCCall(ctx *NodeContext, msg proto.Message, method string) (proto.Message, error) {

Should be changed to this:

func RPCCall[Req, Resp proto.Message](ctx *NodeContext, req Req, method string) (Resp, error) {

This will allow callers (generated code) to do this:

func ReadRPC(ctx *NodeContext, in *ReadRequest) (resp *ReadResponse, err error) {
	return gorums.RPCCall[*ReadRequest, *ReadResponse](ctx, in, "proto.Storage.ReadRPC")
}

Instead of this:

func ReadRPC(ctx *NodeContext, in *ReadRequest) (resp *ReadResponse, err error) {
	res, err := gorums.RPCCall(ctx, in, "proto.Storage.ReadRPC")
	if err != nil {
		return nil, err
	}
	return res.(*ReadResponse), err
}