More explicit communication about the type of data

[cholmes]

In discussion on a stac-extensions issue it's become clear that there's a desire to be able to communicate things like 'this data is from a satellite'. The presence of an extension in stac_extensions sorta can be used in that way, but in that issue we decided that it's not an explicit relationship - the presence in stac_extensions should just be used to communicate that it should be used as validation.

There seem to be a few extensions where this could make sense - satellite (and potentially aerial, drone, balloon in the future), sar, pointcloud and raster.

There's probably a few different ways to do this, can use this issue to discuss ideas. But it seems like a 'platform type', an 'instrument type', and a 'data type' could potentially make sense somewhere. Could start out with each extension using a common field, and then migrate to common metadata at some point. We could use the appropriate extension and just add a single required field like 'platform_type=satellite', instead of requiring 'one of' the fields.

cc @jjrom

[m-mohr]

This could be a separate extension, maybe as part of the ARD initiatives. It seems STAC can't define these kind of vocabularies. Maybe use the themes extension?