Add support for providing a function as the key

[toonetown]

It would be helpful to be able to provide a function or a string as the key to the verify functions. Such a function would be passed the value of kid from the header as a single parameter, and would return a string to use as the key. This would simplify, for example, using a hash table or a Redis database for looking up keys based off of the kid parameter.

I actually have some code that does this, and am willing to put together a pull request if it would be something that is desired.

[SkyLothar]

oh, that's a good idea.
now, i'm doing following steps to perform a kid validaiton

• load jwt to create a jwt_obj
• find out the right key according to the kid
• verify the jwt_obj

yes, a kid validation function sounds good.
much appreciated if you send a pr!

[toonetown]

OK - I'll get it worked up probably tomorrow and get something submitted for review.