Require title

[emmanuelmathot]

Following closure of #1232 and the crosswalk between OGC API - Records and STAC, this issue aims at specifying a mandatory title in STAC 2.0

title

title is required in Records for Collections and Items. Having titles in the resources and also in the links makes the browsing experience much nicer (see STAC Browser). Titles in Catalogs/Collections and their corresponding links should be aligned anyway. So I'm wondering whether we should require the title field in Catalogs and/or Collections and/or Items. Collections should always have a title, I think. It would also be good for Catalogs to explicitly choose a title and people use the id as title pretty often aynway. For Items real titles are often not available (e.g. for satellite captures), but anyway a default title provided by the provider would be good for UX.