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

Commits on Aug 19, 2021

Added Auth#createCustomTokenWithOptions to allow token expiry to be specified
rhodgkins
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Increased minimum expiry for custom token to 5 mins (as per https://github.com/firebase/firebase-admin-node/issues/1016#issuecomment-846305419)
rhodgkins
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