Add a method to manually refresh the JWKS

[MicahParks]

This new method should accept a boolean argument, ignoreRateLimit. If set to true, the method will refresh the JWKS ignoring the rate limit. If set to false, it would behave as a channel send currently does for *keyfunc.JWKS's refreshRequests field.

[MicahParks]

Maybe it'd be best to make the method's argument a struct instead of a bool, since it's possible more options may be desirable in the future.