Adding the optional parameter clock_skew_in_seconds=60 to the call to google.oauth2.id_token.verify_token now allows for the token-issuing server's clock to be off by up to a minute without the token becoming invalid due to a 'issued-at-time' timestamp that is in the future.

This option value is used for the token verification instead of the fixed 60 seconds from
the earlier commit.

This way, the user of firebase_admin can decide if he/she wants to set that value or not.
Also all existing uses of firebase_admin won't suddenly change behaviour, since if the
option is not specified, it's default of 0 is equivalent to what was used before the
introduction of the new option.

Fschaeck clock skew