spec: Add ObjectType with CustomType

Closes #25

This breaking change ensures there is consistency that specification implementations can use CustomType with all types, in this case object types underlying any attribute type.

For existing implementations, such as:

{
            "name": "list_object_attribute",
            "list": {
              "computed_optional_required": "computed",
              "element_type": {
                "object": [
                  {
                    "name": "obj_string_attr",
                    "string": {}
                  }
                ]
              }
            }
          },

Are now defined as:

{
            "name": "list_object_attribute",
            "list": {
              "computed_optional_required": "computed",
              "element_type": {
                "object": {
                  "attribute_types": [
                    {
                      "name": "obj_string_attr",
                      "string": {}
                    }
                  ]
                }
              }
            }
          },

Note that there is now a slight inconsistency between how CustomType is defined on collection types (ListType, MapType, and SetType) that are already element types and need to include their own element type. The CustomType is specified along side the element type, rather than explicitly having an ElementType delineation.

Currently:

          {
            "name": "map_list_string_attribute",
            "map": {
              "computed_optional_required": "computed",
              "element_type": {
                "list": {
                  "custom_type": {
                    "import": "github.com/hashicorp/terraform-plugin-framework/types/basetypes",
                    "type": "basetypes.ListType",
                    "value_type": "basetypes.ListValue"
                  },
                  "string": {}
                }
              }
            }
          },

It may make sense to reintroduce what was originally designed in the specification with the verbose ElementType property for clarity.

          {
            "name": "map_list_string_attribute",
            "map": {
              "computed_optional_required": "computed",
              "element_type": {
                "list": {
                  "custom_type": {
                    "import": "github.com/hashicorp/terraform-plugin-framework/types/basetypes",
                    "type": "basetypes.ListType",
                    "value_type": "basetypes.ListValue"
                  },
                  "element_type": {
                    "string": {}
                  }
                }
              }
            }
          },

hashicorp / terraform-plugin-codegen-spec

spec: Add ObjectType with CustomType #26