feat(base_node_grpc_client): add getActiveValidatorNodes method (#4719)

Description
---
Adding the `getActiveValidatorNodes` method to `base_node_grpc_client` JS client.

Motivation and Context
---
For the `tari-explorer` web app (and other future usages, like cucumber integration tests), we want to retrieve a list of the active validator nodes from the base node JS client.

The method is already implemented in the base node itself, this PR is only to allow calling it from the client.

How Has This Been Tested?
---
Manually from a modified version of the `tari-explorer` web app

clients/base_node_grpc_client/src/index.js

@@ -42,7 +42,8 @@ function Client(address = "127.0.0.1:18142") {
     "getTipInfo",
     "searchUtxos",
     "getTokens",
-    "getNetworkDifficulty"
+    "getNetworkDifficulty",
+    "getActiveValidatorNodes",
   ];
   methods.forEach((method) => {
     this[method] = (arg) => this.inner[method]().sendMessage(arg);