Clean up Cosmos DB types

[ejizba]

I'm working on updating the docs for Cosmos DB here and noticed a few things that we need to clean up:

1. I originally thought the trigger & input options had a lot more in common, but turns out there's several differences. I've separated the two interfaces instead of one extending the other.
2. An input binding and output binding can use a single document or an array of documents, so we should use unknown for the type instead of assuming it's an array.