Load/Store API

[ax3l]

While we add more structure to PALS files (pals-project/pals#152 pals-project/pals#153), I suggest we move the load/store logic into official PALS APIs.

What I mean is, simple file load/save logic like this:
https://github.com/BLAST-ImpactX/impactx/blob/25.12/src/python/impactx/extensions/KnownElementsList.py#L30-L51

Could be free functions in PALS Python. That way, we can add logic and selectors on top of the pure Pydantic logic that we implemented so far here.

Alternatively, we can also implement this as methods on Beamline.