MOVEONLY: Move wallet backup and encryption RPCs out of rpcwallet

Summary:

As part of an effort to split rpcwallet as per #23622, this moves rpcdump.cpp into the new wallet/rpc directory as well as moving backup and encryption RPCs out of rpcwallet.

This is a backport of core#23647

Depends on D12777

Note the differences introduced by D45.In the spirit of that diff, I added a Span<const CRPCCommand> GetWalletEncryptRPCCommands() function in encrypt.cpp rather than using forward declarations in rpcwallet.cpp.

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12778