Count Extension: endpoint /count to get only a count for a query

[philvarner]

Moved from:

• Count Extension: endpoint /count to get only a count for a query radiantearth/stac-api-spec#297

Related:

• Collection counts in spec? radiantearth/stac-api-spec#355

Same as /search, but instead gives a count of the results rather than the result bodies.

Should have a parameter for "exhaustive" results, since most datastores only give an estimate by default (e.g., this is what Elasticsearch does)