One method on IHttpClient for get, post etc

Here's the next one.
This change reduces `IHttpClient.Get(GetRequest request)` and `IHttpClient.Post(PostRequest request)` to one method, `IHttpClient.Send(Request request)` where  `Request.Method` identifies it as `Get`, `Post` or other.

This means that `IHttpClient` supports multiple http verbs and no further changes to this interface are needed if the implementation needs to change to better support verbs beyond get and post.

I am aware that this is a breaking change to clients that implement their own `IHttpClient` but this is necessary for further work on http verbs. I also think that updating a wrapper `IHttpClient` implementation will not take long at all.

`GetRequestHander` and `PostRequestHandler` now look very similar and are what limit us to Get and Post only. They are next.