Add getBalance to graph-ts (#1584)

* Add getBalance call to graph-ts

* Add changeset

Author: incrypto32

.changeset/four-baboons-ring.md

@@ -0,0 +1,5 @@
+---
+'@graphprotocol/graph-ts': minor
+---
+
+Added getBalance call to ethereum API

packages/ts/chain/ethereum.ts

@@ -5,6 +5,7 @@ import { Address, BigInt } from '../common/numbers';
 /** Host Ethereum interface */
 export declare namespace ethereum {
   function call(call: SmartContractCall): Array<Value> | null;
+  function getBalance(address: Address): BigInt;
   function encode(token: Value): Bytes | null;
   function decode(types: String, data: Bytes): Value | null;
 }