app_check.verify_token can also raise PyJWKClientError, but that is not documented

[dmelo]

The documentation of app_check.verify_token states that the method can raise ValueError. But looking into the documentation, we can see that it can also raise an PyJWKClientError.

I believe we should add something like:

PyJWKClientError: If PyJWKClient fails to fetch the signing key

Relevant Code:

from https://github.com/firebase/firebase-admin-python/blob/f0865f7493a2c642d2e551efdf19da1e53c1a8c3/firebase_admin/app_check.py#LL38C1-L38C1:

    Raises:
        ValueError: If the app's ``project_id`` is invalid or unspecified,
        or if the token's headers or payload are invalid.