Add support for maps based on `additionalProperties` field

Context

RFC: https://docs.google.com/document/d/15PJokoO7mkMYsc2Z7DN1copykQgH5Kt7nnzb-mcNcTo/edit#heading=h.4syvri1n60bk

References:

This PR adds support for:

MapNestedAttribute
MapAttribute
MapType (only possible under a collection type, ListAttribute, SetAttribute, or MapAttribute)

I also added documentation to the design doc and added unit/integration tests to cover since this there aren't any use-cases in existing OAS tests.

Details

additionalProperties indicates that key names in a schema can be any string, with a value defined by the schema defined in additionalProperties.type.

Examples

This works for simple maps that have primitive element types (MapAttribute)

// Keys can be any string, value can be any string (map of strings)
{
  "type": "object",
  "additionalProperties": { 
    "type": "string" 
  }
}

This also works for more complex maps that have objects (MapNestedAttribute)

// Keys can be any string, value can be an object that matches the schema
{
  "type": "object",
  "additionalProperties": { 
    "type": "object",
    "properties": {
        // ... define object for value of map
    }
  }
}

Finally, it can also exist as a MapType in an array:

// Array with items, keys can be any string, value can be any string (map of strings)
{
  "type": "array",
  "items": {
    "type": "object"
    "additionalProperties": {
      "type": "string"
    }
  }
}

I wrote integration tests to cover this specific functionality as the existing OAS do not cover this use-case 👍🏻

Notes:

It's worth noting that the usage of additionalProperties will override any existing properties that are set. In JSON schema, additionalProperties is "in addition" to the defined properties, however we cannot represent that in a TF Plugin Framework schema.
This override will only happen if a schema is provided in additionalProperties, if additionalProperties: false is found (indicating that NO additional properties are allowed), it will treat the schema like an object.

hashicorp / terraform-plugin-codegen-openapi