[Feature]: Assigning input sandboxes to jobs

### User Story

As a developer, I want `diracx-tasks` to assign input sandboxes to jobs once they reach the router, so that users can easily retrieve the input sandboxes when needed.

### Feature Description

Here is how it's done in DIRAC: https://github.com/DIRACGrid/DIRAC/blob/15976973cb583d0232acf3e481b7bc53173c83c4/src/DIRAC/WorkloadManagementSystem/Executor/JobSanity.py#L43-L67

Here we would rely on `diracx-tasks` to replace the DIRAC Executors.
Note: it requires `diracx-tasks` to be implemented.

### Definition of Done

- [ ] Input sandboxes are assigned by a diracx task
- [ ] Unit tests (general case, unknown input sandbox provided, ...)

### Alternatives Considered

_No response_

### Related Issues

_No response_

### Additional Context

_No response_