Async API to determine project ID

[hiranya911]

Currently the SDK uses the following synchronous procedure to determine the project ID:

1. Get project ID from AppOptions.
2. Get project ID from the service account credentials.
3. Get project ID from the environment variables.

New GCP runtimes like Cloud Run no longer expose the project ID as environment variables. Therefore the SDK does not work in these environments if the project ID is not specified via AppOptions or service account credentials (see b/143090254).

admin.initializeApp();
admin.auth().verifyIdToken(token); // does not work in Cloud Run

To support this we need to get the project ID from the local metadata server. This is an async operation, and therefore we need to turn our project ID discovery logic into an async method.

With this PR I'm starting to refactor the SDK so that it can find the project ID via an async method call. I've wrapped the existing util.getProjectId() method into a new util.findProjectId() method that returns a Promise. I'm also updating the ID token verification logic to use the new async method.

I will refactor the rest of the SDK to use the new async method in future PRs. Finally, we will modify the credentials module to discover the project ID by making a call to the local metadata server.

[lahirumaramba]

LGTM!