add optional numberMatched and numberReturned fields to ItemCollection

[philvarner]

Related Issue(s):

• n/a

Proposed Changes:

1. add optional numberMatched and numberReturned attributes to ItemCollection. These are recommended for a OCG API - Features Part 1 implemenation of ../items and part of OGC Commons now. stac-server and resto already add these to both ../items and /search ItemCollection responses, and doing this in stac-fastapi would be trivial because of existing support for Context Extension. After making this change, I feel we can also deprecate the Context Extension, but that's a separate decision.

PR Checklist:

• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.