feat: Implement verify method

[KKimj]

Related Issues

fixes #20 #33
Thanks to @tkfinitefield

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[rrousselGit]

As mentioned in the original PR, we'd want tests :)

[KKimj]

@rrousselGit

I'm having difficulty designing test scenarios when trying to write test code.
I think testing the process of encoding and decoding a dummy JWT token might be a good approach, but I'm curious to hear your thoughts.

If you have the time, I'd be interested in collaborating on writing the tests.
Thanks!

[labrom]

Looking at what needs to be verified from https://firebase.google.com/docs/auth/admin/verify-id-tokens#verify_id_tokens_using_a_third-party_jwt_library:

• audience: already verified in FirebaseTokenVerifier._verifyContent()
• issuer: already verified in FirebaseTokenVerifier._verifyContent()
• sub: presence already verified in FirebaseTokenVerifier._verifyContent()
• exp: unless I missed something, doesn't seem to be verified, but JWT.verify() will take care of it (this should be documented because for the sake of consistency we would have wanted to verify it in _verifyContent())
• iat: doesn't seem to be verified either, JWT.verify() has the issueAt parameter but the way it handles it and verifies the token's iat confuses me, so I would actually recommend checking it in _verifyContent()

So in summary, the use of JWT.verify() without any parameter in the proposed implementation is consistent with the existing implementation, assuming 'iat' is verified in _verifyContent().

I would recommend removing the existing verifyJwtSignature() function and the commented-out line that invokes it.