initial text for Label changes

[philvarner]

Related Issue(s): #936

Proposed Changes:

1. in Label Extension, use Asset Role instead of Asset name to indicate the labels assets
2. refine language around raster vs. vector labels

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.

[jisantuc]

woops I made these comments the other day but forgot to submit the review. Anyway this looks pretty sensible to me -- I'm open to being convinced about both comments.

[jisantuc]

Suggested change

	##### Asset names
	The Label Extension recommends assets be named with the keys "raster_labels" or "vector_labels". However, this is just a recommendation, and acutal name are entirely at the discretion of the implementer.

kind of the inverse of the previous comment -- I think tooling developers would be better off if there weren't recommended names for things, since the lines between recommended, required, and "required for this one tool to work" aren't always clearly drawn.

[philvarner]

I agree. I only left this in the spec b/c it was the required way to indicate labels previously. I'm fine with removing it, but I'm worried that their might be tools that rely on it? But, maybe better just to remove it now before 1.0.0?