Extend a Views handler to support all entities in Drupal

Problem/Motivation

The module supports only Node ID as an argument to Contextual filter. Let's extend it to support all entities in Drupal.

Proposed resolution

Extend the round_linking_views_data_alter().

Remaining tasks

• Extend the round_linking_views_data_alter().
• Fix module's codebase and remove a dependency from Node ID.
• Rename everything that's related to node.
• Update tests.
• Add a test for the User entity as well.