Add federation API

Add signing keys fed API request

lib/matrix_server_web/federation_client.ex

@@ -0,0 +1,23 @@
+defmodule MatrixServerWeb.FederationClient do
+  use Tesla
+
+  alias MatrixServerWeb.Endpoint
+  alias MatrixServerWeb.Router.Helpers, as: RouteHelpers
+
+  # TODO: Maybe create database-backed homeserver struct to pass to client function.
+
+  @middleware [
+    {Tesla.Middleware.Headers, [{"Content-Type", "application/json"}]},
+    Tesla.Middleware.JSON
+  ]
+
+  @adapter {Tesla.Adapter.Finch, name: MatrixServerWeb.HTTPClient}
+
+  def client(server_name) do
+    Tesla.client([{Tesla.Middleware.BaseUrl, server_name} | @middleware], @adapter)
+  end
+
+  def get_signing_keys(client) do
+    Tesla.get(client, RouteHelpers.key_path(Endpoint, :get_signing_keys))
+  end
+end