MM-32396 Create endpoint for listing all the roles (#16988)

* Create an endpoint for listing roles

* Add function to client model

* Create tests for listing roles endpoint

* Apply code review suggestions

* Restore GetAllRoles app method

* Use AppContext instead of App

* Minor fix

* Refactor according to new changes

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/role.go

@@ -37,6 +37,20 @@ func (a *App) GetRole(id string) (*model.Role, *model.AppError) {
 	return role, nil
 }
 
+func (a *App) GetAllRoles() ([]*model.Role, *model.AppError) {
+	roles, err := a.Srv().Store.Role().GetAll()
+	if err != nil {
+		return nil, model.NewAppError("GetAllRoles", "app.role.get_all.app_error", nil, err.Error(), http.StatusInternalServerError)
+	}
+
+	appErr := a.Srv().mergeChannelHigherScopedPermissions(roles)
+	if appErr != nil {
+		return nil, appErr
+	}
+
+	return roles, nil
+}
+
 func (s *Server) GetRoleByName(ctx context.Context, name string) (*model.Role, *model.AppError) {
 	role, nErr := s.Store.Role().GetByName(ctx, name)
 	if nErr != nil {