This refactor aims to house the CalculateFeeRate function in a more
"shareable" location. It used to be a non-exported function inside
of rpcserver.go. However, other routines, such as FundPsbt inside
of walletkit_server.go, could also rely on the same fee calculation
functionality. So we move the function to rpc_utils.go and export it,
and we will reuse it in the FundPsbt workflow in a future PR.
With the middleware handler in place, we now need to add a new gRPC
interceptor to the interceptor chain that will send messages to the
registered middlewares for each event that could be of interest to them.
