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/state.lua

@@ -235,7 +235,21 @@ end
 M.dependencies = {
   info = { endpoint = "/info", key = "info", state = "INFO", refresh = false },
   revisions = { endpoint = "/mr/revisions", key = "Revisions", state = "MR_REVISIONS", refresh = false },
-  project_members = { endpoint = "/members", key = "ProjectMembers", state = "PROJECT_MEMBERS", refresh = false },
+  project_members = {
+    endpoint = "/project/members",
+    key = "ProjectMembers",
+    state = "PROJECT_MEMBERS",
+    refresh = false,
+  },
 }
 
+-- This function clears out all of the previously fetched data. It's used
+-- to reset the plugin state when the Go server is restarted
+M.clear_data = function()
+  M.INFO = nil
+  for _, dep in ipairs(M.dependencies) do
+    M[dep.state] = nil
+  end
+end
+
 return M