Added a new structure to the '@graph' section of ldjson

Problem/Motivation

Since the advent of AI, my clients have regularly asked me to modify their ldjson structure, particularly the '@graph' section.

Proposed resolution

The solution I propose is to implement an `ALTER` hook so that we can modify the table containing the information, allowing us to extend it using custom modules.
Within the static parseJsonld method, users can implement the hook_schema_metatag_items_alter hook as they see fit.

You'll find my patch, which implements this mechanism.