Allow adding metadata to webcams

[dbrgn]

In v15, there should be a way to specify webcams with metadata (e.g. location, description, etc) similar to sensors.

Maybe we could add a new field (webcams? cams? cameras?) that replaces the cam field and is a list of objects?

Use case: We have a web cam connected to the 3D printer in our space (https://webcam.coredump.ch/, currently broken but will be fixed soon™), and would like to label it appropriately.