Add an Item to a collection / catalog

[cholmes]

A very useful little command would be to take an Item and add it to a given collection. The merge command can now add a child catalog / collection to a parent. This would be similar, but would just be adding a single item, and then update the various links (collection, parent, etc). This would help with using rio stac, as it just creates items, and then stactools can let you add those to catalogs / collections. And it's also useful for rearranging a catalog.