extension to facilitate faceted search

[pvgenuchten]

Initial work on #52

Current implementation uses terminology inspired by Elastic API. Since it is a common catalogue backend.

The work may also be relevant for ogc-api-common (ogc-api-features, ...). However faceted search is currently more common in catalogues.

[pvretano]

Paul, would it be too much trouble to move this to the proposals directory? I want to keep the extensions directory reserved for extensions that the SWG is tasked to work on in the charter like the collections extension. Extensions like faceted search are proposed extensions and we will need to revise the charter before that SWG can "officially" work on them. Once we get close to completing core and the collections extension, we can discuss additional extensions such as the ISO 19XXX extension, transactions, harvesting and of course faceted search.
If merging the PR first would make the move easier then go ahead and do that.

[pvgenuchten]

moved to proposals