Implement KOOK provider

[gehongyan]

This issue is created to track the work of adding support for the KOOK OAuth2 to this AspNet.Security.OAuth.Providers project.

The KOOK OAuth2 documentation can be found here:

https://developer.kookapp.cn/doc/oauth2 (Simplified Chinese only)

The source code of the documentation can be found here:

https://github.com/kaiheila/api-docs (Simplified Chinese only)

Pages related to OAuth2 are listed here:

OAuth2 Intro: https://github.com/kaiheila/api-docs/blob/main/docs/zh-cn/oauth2.md (Simplified Chinese only)
OAuth API: https://github.com/kaiheila/api-docs/blob/main/docs/zh-cn/http/oauth.md (Simplified Chinese only)

[martincostello]

Thanks for your contribution - the new provider is now available from NuGet.org.