Make a request factory field for keyfunc.Options

[MicahParks]

When making a request to a JWKS endpoint, sometimes a package user would like to do non-standard things, like include a User-Agent header.

Make a new field on keyfunc.Options that is a function with the signature RequestFactory. If that function is non-nil, use that function to make requests for the JWKS endpoint.

I believe this new field would overwrite the context.Context related field as well as the URL related field.