Provide a server impl of the new Puppet http client

Currently Puppet Server implements a wrapper around the Apache AsyncHttp library (with help from the puppetlabs/clj-http-client library).

Coremmunity has reworked the http client in Puppet and the contract that extensions should implement (like there is one now).

We should create a new HttpClient in Puppet Server that implements the new interface and if the new interface is available we should configure Puppet with it.

Here are some notes from previous discussions:

I was thinking puppetserver could register its client (similar to the existing `Puppet::Server::HttpClient`) but with the following differences:

• A Puppet::Server::HttpClient object is created for a given server/port/use_ssl, and later the request path/query parameters/headers are passed to the get/post method. With the new API, the get/post methods receive the full URL.
• The existing Puppet::Server::HttpClient maps HTTP response codes back to Net::HTTP, but that's no longer necessary. Puppetserver can create a Puppet::HTTP::Response object directly with the status code, message, etc. and return that (or a subclass of it)
• The client can raise `Puppet::HTTP::HTTPError` or more specific subclasses if it wants instead of deriving from SocketError
   
  Example implementation registration: https://gist.github.com/joshcooper/98344933dbd8c44d63acd9183e1481b3

 
Whoever picks this up should reach out to Josh Cooper for further details.