Support access via IAM credentials

Simply, the documentation in the Readme states that this library requires 'A Google Maps API key.', but most other services I've used (like Storage, for example) allow and default to using IAM credentials supplied via environment variable GOOGLE_APPLICATION_CREDENTIALS. When enabling the Places API through console, it lists 3 forms of valid authorization - API key, OAuth (for accessing user data), and IAM. Since the console supports IAM access, shouldn't this library?

So the request is either to support this, or to update the readme if it is just supported somehow.

從 README 中說明該函式庫需要 Google Maps API 金鑰的部分開始，然後檢查該函式庫文件中記載的授權選項。確認是否支援 GOOGLE_APPLICATION_CREDENTIALS 和 IAM 憑證；完成的標準是加入有文件說明的支援，或更新 README 以準確描述此限制。

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