Update jwt-cpp to 0.5.0+

[DrDaveD]

jwt/cpp 0.5.0 has several incompatibilities, so it will take some effort to upgrade.

[duncanmmacleod]

Pinning to jwt-cpp 0.4.* is now a problem on conda-forge, has there been any progress on this @DrDaveD @djw8605?

[DrDaveD]

Nobody is working on it as far as I know.

[olifre]

I've now packaged jwt-cpp-0.6.0 for Gentoo, and trying to compile scitokens-cpp errors out in various places.
So this will definitely also require code changes.

[amadio]

I just bumped jwt-cpp to 0.7.0, which introduced more breaking changes, see https://bugs.gentoo.org/923309 and https://github.com/Thalhammer/jwt-cpp/releases/tag/v0.7.0.