[Beyonce]: Support inverted indexes

[jcarver989]

This PR adds support for creating an "inverted index" -- which is a particular flavor of a GSI.

You can click the link above for more context, but the basic idea is it's often useful to flip your primary table's partition and sort keys in a GSI. Often this "unlocks" several new query patterns -- e.g. querying the "other side" of a relationship.

Previously, when defining a GSI we wouldn't allow you to use "key fields" on a model (i.e. a field that is synthesized when you call FooModel.create({ ... })). Now we allow you to specify your table's partition key or sort key.

A nice future improvement that's out of scope for this PR would be to split model definitions into "data fields" and "key fields". This would allow for defining arbitrary fields on your model that would be automatically synthesized when you call .create({ ... }). You can think of "partitionKey" and "sortKey" as being special cases of that idea -- e.g.:

        Item:
          keys:
            partitionKey: [Item, $id]
            sortKey: [Item, $id]
            index1PartitionKey: [User, $creatorId]
            index1SortKey: [Item, $createdAt, $id]
          fields:
            id: string
            creatorId: string
            createdAt: string // ISO date string
            jsonData: string
GSIs:
  index1:
    partitionKey: $index1PartitionKey
    sortKey: $index1SortKey

const item = Item.create({ 
  id: "...", 
  creatorId: "...", 
  createdAt: "...", 
  jsonData: "..." 
})  // all the keys get auto-appended