[FR] Add return type annotations for public functions (firestore.client(), storage.bucket(), etc)

**Is your feature request related to a problem? Please describe.**
VSCode can use Python type annotations to provide a much better experience, and allow completion.

**Describe the solution you'd like**
function signatures should return a return type, to allow completions. For example, in firestore.py
`def client() -> google.cloud.firestore.Client` would allow completion on client().collection("something"), etc.

**Describe alternatives you've considered**
As a workaround, I'm using the following today:
```
from firebase_admin import firestore
from google.cloud.firestore import Client as FirestoreClient # importing the return type of firestore.client()
db: FirestoreClient = firestore.client() # Adding an annotation to my db variable, to enable completion and syntax checking
```

**Additional context**
Using the latest VSCode, with the Pylance extension.