[google_identity_services_web] - Enhance maybeEnum to handle if enum value is not exist with a return of null.

[Hamza-Nasser]

The old maybeEnum was throwing a transitive ArgumentError that is being thrown from the extension function EnumByName::byName if the string value does not match any, now it is returning null instead.
This PR also have a new test file for the maybeEnum function with different test cases.

Fixes flutter/flutter#166548

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