Implement semantic JWT verification rules

This work should hold off until the ticket here is closed https://github.com/TBD54566975/web5-rs/issues/144

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Currently we verify JWT's only on the cryptographic signature, but we have a set of semantic verifications which must be true for verification to pass. For inspiration, see all the set of `if` conditions in `web5-js` [linked here](https://github.com/TBD54566975/web5-js/blob/1d59f714592debb7b7fd0f38e929328fb221391b/packages/credentials/src/jwt.ts#L143).