Improve the CAD->TGeo integration path for o2-sim with a
configurable JSON-driven setup for externally provided CAD geometry.
The configuration can describe multiple passive external modules and
sensitive external detectors, both activated through the normal
detector/module list.

Add a --extGeomFile option carrying the external geometry
configuration. Passive CAD modules no longer need to be hardcoded in
build_geometry.C, and several configured CAD modules can be injected
in one simulation.

Introduce shared CAD geometry utilities to JIT-load
O2_CADtoTGeo.py-style ROOT macros into unique namespaces, allowing
several CAD modules to coexist in one Cling session despite identical
exported builder symbols. The utilities also remap imported CAD media
into the O2 MaterialManager.

Add o2::ext::ExternalDetector, a generic sensitive detector
implementation for externally supplied CAD geometry. Configured
volumes or media can be marked sensitive, the detector is tied to a
free DetID slot, and hits flow through the standard o2-sim forwarding
and merging machinery.

Add o2::ext::Hit as a detector-agnostic external hit payload storing
entrance/exit position, momentum, energy, energy loss, time, length,
PDG code and track status flags. All external detector instances share
this wire format, so the hit merger only needs one external hit type.

Support optional per-detector sensitive actions loaded from ROOT
macros at runtime via GetFromMacro. When no custom action is
configured, a built-in charged-track entrance/exit action is used.

Register active external detectors in O2HitMerger so their hits are
persisted in parallel mode, and key DetImpl hit collection buffers by
detector instance to support multiple detectors sharing the same C++
type.

Add a self-contained SimExample with two artificial sensitive external
detectors, demonstrating configurable geometry injection, built-in and
custom sensitive actions, and parallel-mode hit persistence.