Simplify Go Endpoints + Add Tests (#120)

This MR represents a major refactor of the Go codebase, as well as introducing tests for the handlers. The MR also introduces an endpoint to shutdown or restart the Go server, which may be useful for clients who want to refresh the state of the plugin after checking out branches. Finally, this MR adds a contributing document for users who want to make feature changes.

lua/gitlab/async.lua

@@ -29,12 +29,13 @@ function async:fetch(dependencies, i, argTable)
 
   local dependency = dependencies[i]
 
-  -- Do not call endpoint unless refresh is required
+  -- If we have data already and refresh is not required, skip this API call
   if state[dependency.state] ~= nil and not dependency.refresh then
     self:fetch(dependencies, i + 1, argTable)
     return
   end
 
+  -- Call the API, set the data, and then call the next API
   job.run_job(dependency.endpoint, "GET", dependency.body, function(data)
     state[dependency.state] = data[dependency.key]
     self:fetch(dependencies, i + 1, argTable)
@@ -54,11 +55,13 @@ M.sequence = function(dependencies, cb)
       end
     end
 
+    -- If go server is already running, then start fetching the values in sequence
     if state.go_server_running then
       handler:fetch(dependencies, 1, argTable)
       return
     end
 
+    -- Otherwise, start the go server and start fetching the values
     server.start(function()
       state.go_server_running = true
       handler:fetch(dependencies, 1, argTable)