Integration with TMGMT XTM project

Problem/Motivation

Restore compatibility code with TMGMT XTM → module.

Earlier builds of this project supported adjustments for the XTM module:
- Adjusting module weights
- Extending and changing Cart object behavior
- Additional hooks

However since the generic use case of TMGMT Partial works w/o these specific modifications, the code was removed to avoid that dependency.

Proposed resolution

Restore the overrides and customizations for XTM as a sub module of this module.