add option for additional checks

[grounded042]

At times consumers of the package want to validate more things in a token than what this package provides. The go to method of doing this is via the ValidatonKeyGetter func. We should provide specific functionality to do these checks instead of having users tack things into the key getter func.

[sergiught]

We just released the v2.0.0-beta 🥳 !

You can start testing it by running go get github.com/auth0/go-jwt-middleware/v2@v2.0.0-beta.

In case of issues fetching the v2 you might want to try go clean --modcache first before doing go get.

I'm closing this issue as now this is part of v2, but feel free to reopen if needed.