Py-identity-model

* Change `HttpClient` extensions methods to rather extend `HttpMessageInvoker` - thanks https://github.com/IdentityModel/IdentityModel2/pull/130
* Add support for [OAuth Device Flow](https://tools.ietf.org/wg/oauth/draft-ietf-oauth-device-flow/)
* support for reading discovery document
* extension method for device authorization requests
* extensions method for device token request

* HttpClientFactory-friendly OAuth and OIDC client libs & discovery document cache
* SourceLink support
* Updated protocol constants

Blog post:
https://leastprivilege.com/2018/06/18/making-the-identitymodel-client-libraries-httpclientfactory-friendly/

More samples:
https://github.com/IdentityServer/IdentityServer4.Samples/tree/dev/Clients

Bug fixes for `AccessTokenDelegatingHandler` and `RefreshTokenDelegatingHandler`

* Added delegating handlers for client credentials and refresh token scenarios for better composability.
* Simplified dependency graph and use "in-box" HttpClient wherever possible