Add support/mounting structure to detector geometry description

[bryngemark]

A lot of our detector components hang in the air. Or, is that still vacuum? 🤔 This doesn't really contribute to the realism of the radiation environment in which we're trying to identify DM.

Describe the solution you'd like For better realism when it comes to scattering and all kinds of junk, it would be great to

• have some rudimentary implementation of tracker plate, TS plastic mounting, target mounting, ...
• and also use air around it if we don't already.

Describe alternatives you've considered To live my life in an ideal world. Tempting

[tvami]

This is partially handled in https://github.com/LDMX-Software/ldmx-sw/pull/1514

The rest would need to be done by somebody else who has access to model with the information