Add numberReturned and numberMatched fields to ItemCollection responses

[philvarner]

stac-fastapi advertises OAFeat as a conformance class, but doesn't include numberReturned or numberMatched in the .../items endpoint response. These are optional fields, but they should be added if the context extension is enabled, since these fields are in the context object as returned and matched.

This PR

• add optional numberMatched and numberReturned fields to ItemCollection radiantearth/stac-api-spec#338

added these as optional fields to any ItemCollection response, so this should be added to /search

[gadomski]

@philvarner these aren't required, though, correct? I know that the pgstac backend in particular does not always populate context, since it's expensive.

Should this issue be reworded "Add numberReturned and numberMatched fields ... if the context extension is enabled"?

[philvarner]

Correct, they are not required. I used "should" in the technical sense of "ought to", rather than "must" / "shall". I rewrote to be more clear.