No ParserOption for requiring nbf claim

[equalsgibson]

Description

Reviewing the current ParserOptions, there are many methods to check optional registered claim values (such as WithExpirationRequired()).

It would appear that the Not Before value is checked currently, but there is no supported way to "require" the not before value like there is with other registered claims.

Current behaviour

Currently, if the nbf claim is not present the default parser will not raise a token validation error.

NOTE: The default parser currently DOES return an error if the nbf claim is present and it is invalid (i.e. wrong format, invalid time).

Expected behaviour

I want to require the nbf claim and return the error jwt.ErrTokenRequiredClaimMissing if it is not present.

[oxisto]

Makes sense. We might need to split adding the functionality and providing more tests in your PR though