This commit moves the ExternalSigner class and RPC methods out of the wallet module.
The enumeratesigners RPC can be used without a wallet since #21417.
With additional modifications external signers could be used without a wallet in general, e.g. via signrawtransaction.
The signerdisplayaddress RPC is ranamed to walletdisplayaddress because it requires wallet context.
A future displayaddress RPC call without wallet context could take a descriptor argument.
This commit fixes a rpc_help.py failure when configured with --disable-wallet.
