Add Bundling Function

[theory]

It might be useful to add a function that can bundle multiple schemas into a single schema, as described here. This pattern allows one to use a single schema JSON object that bundles together all of its subschemas. They're simply collected in the $defs object. A couple of notes:

• The $id of each sub-schema must also be its key in the $defs object.
• Probably makes sense to reject sub-schemas without an $id as an error.

Sample from the docs "customer" schema that bundles the address schema it references:

{
  "$id": "https://example.com/schemas/customer",
  "type": "object",
  "properties": {
    "first_name": { "type": "string" },
    "last_name": { "type": "string" },
    "shipping_address": { "$ref": "/schemas/address" },
    "billing_address": { "$ref": "/schemas/address" }
  },
  "required": ["first_name", "last_name", "shipping_address", "billing_address"],
  "$defs": {
    "https://example.com/schemas/address": {
      "$id": "https://example.com/schemas/address",
      "type": "object",
      "properties": {
        "street_address": { "type": "string" },
        "city": { "type": "string" },
        "state": { "type": "string" }
      },
      "required": ["street_address", "city", "state"]
    }
  }
}