[18.0][ADD] fieldservice_subcontracting

[edescalona]

@BinhexTeam

T22502

The module integrates Field Service with Purchase to support subcontracted Field Service Orders.

It allows companies to assign an FSO to an external worker, automatically create a draft Purchase Order for the subcontracted service, update delivered quantities from FSO timesheets, and navigate between the FSO and the related Purchase Order.

Main features:

1. Mark Field Service workers as subcontractors.
2. Require subcontractor workers to be linked to vendor partners.
3. Configure a subcontracting service product on Field Service order templates.
4. Create draft Purchase Orders from configured Field Service stages.
5. Update Purchase Order quantities and received quantities from FSO timesheets.
6. Add smart buttons to navigate between FSOs and Purchase Orders.
7. Protect reassignment and cancellation flows when active subcontract Purchase Orders exist.
8. Prevent automatic PO cancellation when related vendor bills are already posted.

Typical workflow:

1. Configure a vendor partner and mark the related Field Service worker as subcontractor.
2. Configure the subcontracting service product on the Field Service template.
3. Assign the server action that creates subcontract POs to the desired Field Service stage.
4. Assign the server action that updates delivered quantities to the closing stage.
5. Assign a subcontractor worker to an FSO and move it through the configured stages.
6. Review, confirm and bill the generated Purchase Order using the standard Purchase flow.

The module keeps Purchase Orders under the standard Odoo purchase workflow: generated POs are created as drafts and must be reviewed and confirmed manually.