Option to generate summaries for a catalog/collection in CLI

[cholmes]

It'd be great if it was much easier to generate summaries. This is tracked in pystac at stac-utils/pystac#178

In the command line I could see a dedicated 'generate summary' command, that takes a catalog or collection as an argument and updates it to have a summary of all its child elements. It should have an option to generate 'all' (at least the ones that make sense), or take a list of the properties to summarize.

I think it could also make sense to add it as an option to some of the commands, like copy and merge.

We might even consider making summaries default to true, as it's a good practice. Probably should see how they actually work first, as I worry a bit about generating really large summary lists. But once we get some sensible rules in for when/how to generate the summaries (like don't do them if it's more than 5 or 10 distinct values in a property, or only do 'known' STAC extension properties by default) then perhaps we make them default.