feat(modules/nimbus-eth2): Add web3-signer-url option

Author: MartinNikov

modules/nimbus-eth2/args.nix

@@ -106,6 +106,13 @@ with lib; {
     };
   };
 
+  web3-signer-url = mkOption {
+    type = types.nullOr types.str;
+    default = null;
+    example = "http://localhost:9000/";
+    description = mdDoc "Remote Web3Signer URL that will be used as a source of validators.";
+  };
+
   web3-urls = mkOption {
     type = types.listOf types.str;
     default = [];