Clarified stac_extensions

[m-mohr]

Related Issue(s): #1077

Proposed Changes:

1. For context see stac_extensions and summaries #1077, but it basically makes it more consistent that stac_extensions also allow to list extensions that are used in collection summaries.
2. Re-phrased the stac_extensions description in all specs.
3. Removed the wording around "multiple affected parts" as that should be clear from each individual stac_extensions description, which requires to list stac_extensions if they are implemented. That was mostly for the commons extension back in the days, but nowadays had no real good example anymore. The data cube example in collections for example was not really correct.

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.

[emmanuelmathot]

Shouldn't we add in the collection spec a clarification that explains that stac_extensions in collection should not declare extensions fields that are only summarized as in example collection https://github.com/radiantearth/stac-spec/blob/master/examples/collection-only/collection.json

[m-mohr]

Good point @emmanuelmathot, although I'm not sure whether we should add extensions to stac_extensions that are summarized in colelctions or not. I think best would be to add them, but then we need to change examples.

[m-mohr]

DON'T MERGE YET!

We should still discuss, whether summaries should have an influence on stac_extensions or not.

[m-mohr]

Agreed on it for future-proofness etc.