Add 'from_items' to Extent

[lossyrob]

This refactors the code in Collection.update_extent_from_items into a static method on Extent so that the logic can be used in the case where a user has items but not (yet) a collection. This is useful if you are building a Collection from a set of Items, in that you can do:

items = ???
extent = pystac.Extent.from_items(items)
collection = Collection(id="collection_id", description="a description", extent=extent)
collection.add_items(items)