WIP: derive http client from service

[pepegar]

This is a work in progress to track the status of HTTP client derivation from service protocol (protocol = service annotated trait). The actual implementation:

• creates a new http object within the generated companion
• puts a HttpClient[F[_]: Sync] inside the http object
• creates a new method on HttpClient for each @http annotated method in the protocol, with all the needed.
• Derive HTTP from [Protobuf|Avro|AvroWithSchema] (like we're doing for RPC).

Some assumptions/questions I've taken while implementing this:

• all HTTP methods should be unary, meaning that they only take a parameter
• how do we treat headers? If they're a static value, we can pass them as params to the @http annotation, to try to leave the method as similar to the RPC definition as possible. Otherwise, i haven't found a solution for that, since it'll make method incompatible with RPC...

Test

To see generated code, I've used this file

import freestyle.rpc._
import freestyle.rpc.protocol._
import freestyle.rpc.internal.encoders.pbd._

@service(Avro) trait GithubService[F[_]] {

  @http(POST, "https://github.com/frees-io/freestyle-rpc/issues")
  def postIssue(id: Int): F[String]

  @http(POST, "https://github.com/frees-io/freestyle-rpc/pr")
  def postPullRequest(id: Int): F[String]

}

And loaded directly in the REPL:

$ sbt client/console
scala> :load test.scala

[L-Lavigne]

Closing this PR as resolved in #552. Thanks @pepegar and @rafaparadela !