[Feature]: Support multi-architecture container images via CVMFS multiarch layout

### User Story

As a DIRAC site administrator deploying ARM (aarch64) and other non-x86_64 worker nodes, I want DIRAC to automatically resolve the correct container image for the worker node's architecture, so that Pilot-Jobs can seamlessly execute payloads on heterogeneous clusters without manual per-architecture configuration.

### Feature Description

DIRAC currently hardcodes or configures a single container image path (via `ContainerRoot`) that implicitly assumes `x86_64`. With the introduction of ARM resources and the CVMFS `unpacked.cern.ch` support for multi-architecture images, DIRAC needs to:
1. Detect the worker node architecture at runtime (using platform.machine() / uname -m).
2. Map it to the OCI architecture name used by the CVMFS multiarch layout (e.g. `x86_64`: `amd64`, `aarch64`: `arm64`).
3. Resolve the correct image path under the CVMFS .multiarch directory structure: `/cvmfs/unpacked.cern.ch/.multiarch///:`
For example: `/cvmfs/unpacked.cern.ch/.multiarch/arm64/registry.hub.docker.com/library/alma9:latest`
Note: Variant handling is deferred. Initially, DIRAC only resolves by architecture (uname -m), not by variant. Can be added later if needed.

It should affect both code paths that launch containers:
- `SingularityComputingElement (job execution via the `JobAgent`)
- `dirac-apptainer-exec` (Pilot-Job command execution)

It should maintain backward compatibility with the existing `ContainerRoot` configuration option: if `ContainerRoot` is set and the new multiarch path doesn't exist, fall back to `ContainerRoot`.

### Definition of Done

- [ ] Multi-architecture images hosted in CVMFS unpacked are supported
- [ ] DIRAC should pick the right container based on the underlying architecture
- [ ] Unit tests
- [ ] Documentation
- [ ] Backward compatibility (no breaking change)

### Alternatives Considered

_No response_

### Related Issues

First attempt (CVMFS `unpacked` support for multi-architecture did not exist at that time, we are in the process of testing it with LHCb images): https://github.com/DIRACGrid/DIRAC/pull/7589

### Additional Context

_No response_