Split out jwt token into a separate crate

[arseny-bogomolov]

Before this PR

UnverifiedJwt type is useful for use by Witchcraft applications, but it's currently private to the witchcraft-server crate.

After this PR

==COMMIT_MSG==
UnverifiedJwt type is now in a dedicated witchcraft-jwt crate, and has public parse() method
==COMMIT_MSG==

Possible downsides?

This should probably eventually be a separate repo.

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Description

UnverifiedJwt type is now in a dedicated witchcraft-jwt crate, and has public parse() method

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