Add psf model to InstrumentDescription

[ctoennis]

The InstrumentDescription model should have a place to store the upcoming PSFModel in PR #2643 for each telescope, ideally somewhere in OpticsDescription.

Describe the solution you'd like
The instrument module is where we define the InstrumentDescription model (which currently is collected under SubarrayDescription class in ctapipe). It would be useful then to integrate it with the rest of the model, i.e. add a place for it in OpticsDescription, and add serialization so that it can be written and read. A collection of these could be serialized to a new table (one row per telescope), as we do with other OpticsDescription parts (SubarrayDescription.to_table(kind="optical_psf"), or else just in OpticsDescription... ) .

psf : Optional[PSFModel]  = subarray.tel[tel_id].optics.psf

if psf: 
     plot_psf(psf)  # some general function that plots any PSFModel

Additional context
The PSFModel is found in in PR #2643