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>

model/client4.go

@@ -6483,6 +6483,20 @@ func (c *Client4) DownloadJob(jobId string) ([]byte, *Response, error) {
 
 // Roles Section
 
+// GetAllRoles returns a list of all the roles.
+func (c *Client4) GetAllRoles() ([]*Role, *Response, error) {
+	r, err := c.DoAPIGet(c.rolesRoute(), "")
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+	var list []*Role
+	if jsonErr := json.NewDecoder(r.Body).Decode(&list); jsonErr != nil {
+		return nil, nil, NewAppError("GetAllRoles", "api.unmarshal_error", nil, jsonErr.Error(), http.StatusInternalServerError)
+	}
+	return list, BuildResponse(r), nil
+}
+
 // GetRole gets a single role by ID.
 func (c *Client4) GetRole(id string) (*Role, *Response, error) {
 	r, err := c.DoAPIGet(c.rolesRoute()+fmt.Sprintf("/%v", id), "")