Move external signer out of wallet module

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.

test/functional/test_runner.py

@@ -111,6 +111,7 @@ BASE_SCRIPTS = [
     'wallet_listtransactions.py --legacy-wallet',
     'wallet_listtransactions.py --descriptors',
     'feature_taproot.py',
+    'rpc_signer.py',
     'wallet_signer.py --descriptors',
     # vv Tests less than 60s vv
     'p2p_sendheaders.py',