Create a HTTP in .NET tutorial

Searching for "HTTP in .NET" brings up one thing written by us, which is mostly about ASP.NET and HttpClientFactory cruft:

https://docs.microsoft.com/en-us/dotnet/core/extensions/http-client

We should refocus this on HttpClient scenarios. Suggestions:

- REST with various content types.
- Authentication.
- Form posting.
- Maybe some other "advanced" REST/header scenarios too, as more and more APIs are using these.
- the "HttpClient is an IDisposable connection pool" talk and various ways to instantiate/manage a client. (including, as a subsection, the bulk of the current docs.)
- SNAT
- Counters
- Diagnostics/troubleshooting.

We should also make the JSON extensions a more integrated part of the HttpClient docs. It isn't super obvious that these extensions exist and I've got feedback from people trying to figure out some 3rd party libraries that they went to those libraries because they didn't know the JSON extensions existed and were intimidated by integrating S.T.Json with HttpClient.