Add method to retrieve resource Project ID from GoogleCredentials

[psx95]

Is your feature request related to a problem? Please describe.
GoogleCredentials currently does not provide access to the resource project ID.

It only has method to provide access to the currently used Quota Project ID, which may be different from the resource project ID.

Describe the solution you'd like
A method to retrieve the current resource project ID. Similar methods exists in classes derived from GoogleCredentials such as ServiceAccountCredentials.

Additional context
I'm using this library to retrieve ADCs to authenticate to Google services. Due to the absence of a method to retrieve resource Project ID, the user is required to explicitly set the resource project ID.