Do not require jwks_uri in service discovery

[Wegi]

Feature Request

It should not be required to have the jwsk_uri field mandatory in discovery, as seen in the iOS version of the library: https://github.com/openid/AppAuth-iOS/issues/355

Motivation

Not all providers include the jwks_uri in their documents, and it is not used inside the library.

Description

This is found in the twin library: https://github.com/openid/AppAuth-iOS/issues/355

Alternatives or Workarounds

Manually configuring the Service.