Implement collection summary functionality

[lossyrob]

In #309, the ability to extract collection summary information for extensions was implemented for the eo extension (see SummariesEOExtension in extensions/eo.py). This should be implemented for each other extension, and tested across all the extensions, as well as pydocs written for that functionality.

Top level sphinx documentation should also be written for the summaries extensions as a pattern.

Summary information should only be used where it makes sense; e.g. if something will be a large range of discrete values that doesn't makes sense to put into a range, then we shouldn't make a summary entry for it.

Thought should be put into how the CommonMetadata works here - should we have a similar extension functionality for getting and setting common metadata summary information?

Extensions this issue potentially applies to:

• datacube (no summary fields according to stac-utils/stac-fields)
• eo (testing) (Update EO extension docs #355)
• file (Removed in Update docs and tests for Version, Label, and File Info extensions #442, no summary fields according to stac-utils/stac-fields)
• item_assets (Collection summaries do not apply to this extension)
• label (Add Label Extension summaries #484)
• pointcloud (Update Point Cloud Extension #558)
• projection (Update docs, tests, summaries for stable extensions #372)
• sar (Update SAR Extension #556)
• sat (Update Satellite Extension #509)
• scientific (Update docs, tests, summaries for stable extensions #372)
• timestamps (Update Timestamps Extension #513)
• version (no summary fields according to stac-utils/stac-fields)
• view (Update docs, tests, summaries for stable extensions #372)