* feat: get issuer from context for device auth
* use distinct UserFormURL and UserFormPath
- Properly deprecate UserFormURL and default to old behaviour,
to prevent breaking change.
- Refactor unit tests to test both cases.
* update example
BREAKING CHANGE:
- op.NewOpenIDProvider
- op.NewDynamicOpenIDProvider
The call chain of above functions did not use the context anywhere.
This change removes the context from those fucntion arguments.
