Implement end-to-end encryption for RPC via a tower middleware.

[ImplOfAnImpl]

Currently, end-to-end encryption is implemented on the ad hoc basis for RPC, e.g. in BlockProductionInterface the functions that accept sensitive data and which can be called via RPC (the "_e2e" ones) have to handle encryption explicitly. Now we'd like to be able to send sensitive data over the wallet PRC as well (#1748) and it's getting ugly. We need to implement e2e encryption for all RPC calls via a tower middleware and get rid of the ad hoc solution.