Test jwt claims are correctly validated

[MegaRedHand]

It seems the crate we are using doesn't check the iat claim. We should test that the claims are properly validated and fail when they're invalid.

We can check the jwt.io webpage to find libraries that support "iat" validation: https://www.jwt.io/libraries?programming_language=rust

[lakshya-sky]

@MegaRedHand I've added tests in main...lakshya-sky:ethrex:jwt-iat-claims-tests, which checks if we reject invalid iats. Could you take a look.

Thanks!