Collection assets to core

[cholmes]

Related Issue(s): #1002

Proposed Changes:

1. Updated Collection spec to include Assets
2. Removed Collection-level extension

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.

[cholmes]

Ok, I think this is ready for review. @m-mohr - I hacked through schema changes, so hopefully I did them right. Thought I got myself into a failing state, but I think I figured it out. But would be good for you to check it over.

I did click on 'this is not a breaking change', because I think it is in a very small way? If implementations said collection-assets for the extension then that will fail with it gone (though that will be true for every extension we move out).

[m-mohr]

I've made a couple of changes:

• Remove extension from Code Owners file
• Align collection and item spec a bit more (i.e. moved assets details to "Additional Fields Information")
• As it was the last spec where it was missing, added h4 headers for "Additional Fields Information" in Items
• Add assets to examples (which may conflict with Examples refresh #955, but then it reminds us to check whether there are collection assets in the examples an add them if not)
• Added to changelog that "No change is required except removing collection-assets from the list of stac_extensions."

Please also review my changes @cholmes and feel free to change things again.

[cholmes]

Cool. Just looked it over, changes look good. A couple comments:

As it was the last spec where it was missing, added h4 headers for "Additional Fields Information" in Items

I think I made these changes in another PR too, but we can just merge here and I'll deal.

Add assets to examples (which may conflict with #955, but then it reminds us to check whether there are collection assets in the examples an add them if not)

#955 got merged - I added #1013 to track this though, as I agree we need to be sure to get them in.