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!

[oikkoikk]

are there any updates?

[labrom]

I couldn't modify this PR so I created a new one: https://github.com/invertase/dart_firebase_admin/pull/54. The changes are mostly copied over except I tried to reuse the existing verifyJwtSignature function. The unit tests verify the 'kid' handling that is added to the verify function.

[rrousselGit]

Closing since there's a new PR