PostRequestHandler uses Authorization header

Apologies for the messy commit history, this was tricky to get right.
Changes are reworking PostRequestHandler to work much more like GetRequestHandler and always do oauth via the Authorization header.
This is a step towards merging/eliminating XXXRequestHandlers and handling all http methods.

Also GetRequestAuthTests and PostRequestAuthTests that call the RequestCoordinator with various get/post singed/unsigned combinations in order to test that they all work.

This requires an api key and secret so to get these to build on TC we also check env vars in SevenDigital.Api.Wrapper.Integration.Tests.AppSettingsCredentials for these values.