Adding x5u support in Store

[yanie-liu]

Hi,
My team is working on a project that involves downloading JWTs from external parties and verifying them using the URL provided in the response header, under "x5u". It would be really nice if the JWT crate Store could also support referencing by x5u as well as by "kid" or key_id. Is this at all possible?

[mikkyang]

I think this would significantly complicate the Store, as it would require the store to fetch the contents of the certificate indicated by the x5u. I will think about adding this as an optional feature.