Feature: added support for Ticket linking

[derpixler]

Adds the Link resource class for managing ticket links via the Zammad REST API.

This change adds support for the Zammad Links REST API, enabling users to programmatically link, fetch, and unlink tickets using the PHP client. This is useful for automation workflows, integrations, and any scenario where related tickets need to be connected (e.g., linking a parent ticket to subtasks or grouping related issues).

This merge combines and improves upon two earlier PRs:

• PR Added: Support for Ticket Linking #58 by chii0815 (initial implementation, includes README docs)
• PR add: links API with passing tests #74 by freyca (improved implementation with tests)

Changes

• src/Resource/Link.php — New resource class with methods:
• get($object_id, $object_type) — Fetch links of a ticket
• add(Ticket $source, Ticket $target, $type) — Link two tickets
• remove(Ticket $source, Ticket $target, $type) — Remove a link
• src/ResourceType.php — Added LINK constant
• test/ZammadAPIClient/Resource/LinkTest.php — Integration tests
• README.md — Added "Linking Tickets" documentation section and LINK entry in the resource support table

API endpoints used

+--------+----------------------+
| Method |       Endpoint       |
+--------+----------------------+
| GET    | /api/v1/links        |
| POST   | /api/v1/links/add    |
| DELETE | /api/v1/links/remove |
+--------+----------------------+

Supported link types: normal, parent, child

Credits

Co-authored-by: chii0815
Co-authored-by: freyca