FR: Allow JSON string to be passed to GOOGLE_APPLICATION_CREDENTIALS

* Firebase Product: FCM

In order to use this library to send push notifications, it is necessary to pass the `services_account.json` file path to the environment variable `GOOGLE_APPLICATION_CREDENTIALS`.
However, allowing the JSON string to be passed directly will also help for certain scenarios where the the secret do not necessary exist in a file.

In fact, the [firebase_config](https://github.com/firebase/firebase-admin-go/blob/b04387eff11f911cf10e9a76f4fcbf517b6c9a62/firebase.go#L176) is parsed this way.

For example:
```go
secret := someSecretStore.Get("GOOGLE_CREDS")
os.Setenv("GOOGLE_APPLICATION_CREDENTIALS", secret)
...
# initialize firebase App
```

Start with firebase.go, especially the firebase_config parsing referenced in the issue, then trace how GOOGLE_APPLICATION_CREDENTIALS is consumed during Firebase App initialization. The change is done when credentials supplied as a JSON string work without a file while existing file-path behavior remains intact; the payload does not name a test file.

Written by the indexing model from the issue text.