Collections get_item could also search if supported

[philvarner]

Prior to 0.4.0, get_item would try to walk the collection to find the item id requested, which on a large catalog was problematic.

In 0.4.0, it checks if Features is implemented, and uses that API to retrieve it.

If that's not supported, it should also check for STAC API - Item Search, and use the collections and ids parameters to retrieve the Item.