Fix Documentation tests / mock FCM server in doc tests

[ywegel]

Currently all Documentation tests are ignored via "no_run", as they all would fail, because there are no real google/FCM credentials supplied. This could either be fixed by mocking the servers with mockito, or by having a valid FCM credentials file in the github runner sercrets

oauth_fcm/src/fcm.rs

Line 33 in 674d2bb

/// ```rust no_run