Add support for setting App Settings

[berndverst]

I'd like to be able to add / update App Settings. This can be accomplished via

POST /api/settings

{"somenewsetting": "somevalue",
"oldsettingtoupdate": "newvalue"}

Similarly, you can delete settings via
DELETE /api/settings/somesettingnamehere

Could you please implement these methods in your API client so that I can use this in GitHub Actions? :)

See:
https://github.com/projectkudu/kudu/wiki/REST-API#settings