Defines TypeScript types for rippled account method requests & responses (#1498)

* account_channels

* account_currencies

* account_info

* account_lines

* account_objects

* account_offers

* account_tx

* gateway_balances

* no ripple check

* respond to comments

* export methods

* fix typos

* respond to comments

* edit BaseResponse to be more specific

src/models/methods/gatewayBalances.ts

@@ -0,0 +1,28 @@
+import { LedgerIndex } from '../common'
+import { BaseRequest, BaseResponse } from './baseMethod';
+
+export interface GatewayBalancesRequest extends BaseRequest {
+  command: "gateway_balances"
+  account: string
+  strict?: boolean
+  hotwallet: string | string[]
+  ledger_hash?: string
+  ledger_index?: LedgerIndex
+}
+
+interface Balance {
+  currency: string
+  value: string
+}
+
+export interface GatewayBalancesResponse extends BaseResponse {
+  result: {
+    account: string
+    obligations?: {[currency: string]: string}
+    balances?: {[address: string]: Balance[]}
+    assets?: {[address: string]: Balance[]}
+    ledger_hash?: string
+    ledger_current_index?: number
+    ledger_index?: number
+  }
+}