fix: validate iat and nbf on payload

[christiandavilakoobin]

If a JWT has a nonumeric iat or nbf value, it throws a FATAL error when validating the field.

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[robocoder]

overlaps with #453

[bshaffer]

I like your implementation better than https://github.com/firebase/php-jwt/pull/453/files because it's simpler - but do you think you could add some tests like in the other PR? If so, please also include the exception message in expectExceptionMessage(...)

[christiandavilakoobin]

Hi @bshaffer! You mean this?

[bshaffer]

@christiandavilakoobin you need to sign the CLA before I can merge this!

[christiandavilakoobin]

@bshaffer it says I signed tje CLA on Jun 19, 2024 08:00 PDT

[christiandavilakoobin]

@bshaffer I just saw I push with another mail. I've signed the CLA with that email too.

[bshaffer]

@christiandavilakoobin thank you! it worked