Automatic anonymous authentication

[fenix-hub]

Related to #171
This PR will implement automatic anonymous authentication for unauthorized requests.

1. If a Firestore request is issued without the Client being authenticated (mail/password, token, etc.) an Anonymous authentication will be attempted automatically.
   

If Anonymous sign-in method is disabled (default option), the request will be skipped and the user will be notified.

If Anonymous sign-in method is enabled (manually set), the Client will be authenticated and the request (and further requests) will be processed.

The authenticated anonymous user can always be logged out with logout() function if needed.

2. A new signal is added in Auth for higher-level communication

# Emitted for each Auth request issued.
# `result_code` -> Either `1` if auth succeeded or `error_code` if unsuccessful auth request
# `result_content` -> Either `auth_result` if auth succeeded or `error_message` if unsuccessful auth request
signal auth_request(result_code, result_content)

[WolfgangSenff]

Just a couple very small things, otherwise looks good.