auth: Allow expiry to be specified in .createCustomToken#1017

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rhodgkins wants to merge 2 commits intofirebase:masterfrom bookcreator:custom-token-expiry

+139-24

Commits on Aug 19, 2021

Added Auth#createCustomTokenWithOptions to allow token expiry to be specified
rhodgkins
committed
Increased minimum expiry for custom token to 5 mins (as per firebase#1016 (comment))
rhodgkins
committed