jwt: context cancellation is not honoured

[axw]

The TokenSource returned from jwt.Config.TokenSource does not pass the context into requests to the token endpoint. Hence requests may block despite the context being cancelled, and furthermore context won't be propagated e.g. for distributed tracing.

I can put up a fix, but I've seen that #769 and #770 were closed as "not planned" without comment. It's unclear to me if that was because the CLA wasn't signed, because the change came in as a github PR, or what.

@seankhliao can you please advise?