Region overlay marks and viewer convenience methods

[bmorris3]

Description

Introducing the RegionOverlay mark, which is similar to but distinct from the FootprintOverlay mark. FootprintOverlay is meant specifically for the Footprints plugin, RegionOverlay is for general use like (1) plotting footprints from fileset queries in loaders, (2) plotting the footprint of one image viewer in another. For the latter, think mast-aladin specifically, but this could equally apply to multiple jdaviz image viewers without matched pan/zoom.

RegionOverlay and the accompanying private methods on ImvizImageView plot sky regions in an image viewer. Region overlays have a selected state, and selecting the mark (1) changes the visual style of the mark, and (2) brings selected regions to the top of the plot so that they're not buried under non-selected regions.

These methods are private and API-only for now, though they could be made public and exposed in the UI in the future.

I will ask @Jenneh and @tomdonaldson for default and selected styles in Missions MAST, and update this PR as needed.

https://github.com/user-attachments/assets/a1aecd2f-033a-4596-af6e-4a0644cd3318

Change log entry

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Checklist for package maintainer(s)

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• [ ] Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• [ ] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?