Require type field in links and assets

[cholmes]

Discussions about radiantearth/stac-api-spec#351 in a call on Dec 12 22 lead to a conclusion that the core stac spec should also require type field in links. It'll be added for stac api spec, since it's very useful for API mechanics to use this, like with an 'accepts' header. It'd be good to align the specs. We might also consider it for 'assets' too, as links and assets are quite similar.

[m-mohr]

This should probably also apply to assets so that assets and links are consistent. Also, usually I find it even more useful to have types for assets.

[m-mohr]

Due to radiantearth/stac-api-spec#413 this is obsolete, at least for OGC API alignment. We can of course still discuss whether we want to require the type in 2.0. In this case pleace leave a comment.

As this has been opened as a reaction to the STAC API changes which have been reverted, I'm closing this for now.