Add raw hash signing (#5423)

* add sign any

* Add RPC signMessage call to JS API

* Add signMessage to JSON RPC docs

* PostSignTransaction -> EthSignMessage

* fix doc typo

* revert incorect naming

js/src/api/rpc/parity/parity.js

@@ -522,6 +522,11 @@ export default class Parity {
       .then(outNumber);
   }
 
+  signMessage (address, password, messageHash) {
+    return this._transport
+      .execute('parity_signMessage', inAddress(address), password, inHex(messageHash));
+  }
+
   testPassword (account, password) {
     return this._transport
       .execute('parity_testPassword', inAddress(account), password);