message: chore(sdk-api): fix coin return type
  BitGoAPI .coin method should declare its actual return type
  Ticket:
diff: @@ -18,6 +18,7 @@ import { createHmac } from 'crypto';
  import * as utxolib from '@bitgo/utxo-lib';
  import {
  AliasEnvironments,
  + BaseCoin,
  BitGoBase,
  BitGoRequest,
  CoinConstructor,
  @@ -31,7 +32,6 @@ import {
  getSharedSecret,
  GetSharingKeyOptions,
  GlobalCoinFactory,
  - IBaseCoin,
  IRequestTracer,
  makeRandomKey,
  sanitizeLegacyPath,
  @@ -307,7 +307,7 @@ export class BitGoAPI implements BitGoBase {
  * Create a basecoin object
  * @param name
  */
  - public coin(name: string): IBaseCoin {
  + public coin(name: string): BaseCoin {
  return GlobalCoinFactory.getInstance(this, name);
  }
  