resolve extents to OGC API - Common, Collections, make optional

[tomkralidis]

Updated extents schema reference. I also made extents optional, seeing that the extents schema itself has no required properties, which may end up allowing things like:

extents:

...just so documents can pass schema validation without added value?

[cportele]

+1 to making extent optional. If there is no extent there is no point in having an empty object.

[tomkralidis]

UPDATE 2021-02-08: since rebasing from @pvretano 's recent updates, this PR now updates the record model and example per #70, and adds a feature collection schema for Records responses.