spec: Nest ListType, MapType, and SetType ElementType as explicit object property

[bflad]

Reference: https://github.com/hashicorp/terraform-plugin-codegen-spec/pull/26

For consistency with collection attribute types which use an explicit element_type object property and object types which use an explicit attribute_types property. An additional benefit is that it further distinguishes where a custom_type may apply.

Previous implementations:

{
            "name": "list_map_attribute",
            "list": {
              "computed_optional_required": "computed",
              "element_type": {
                "map": {
                  "string": {}
                }
              }
            }
          },

Are now defined as:

          {
            "name": "list_map_attribute",
            "list": {
              "computed_optional_required": "computed",
              "element_type": {
                "map": {
                  "element_type": {
                    "string": {}
                  }
                }
              }
            }
          },

The Go bindings and any consuming code do not require changes since Go implicitly required the ElementType field definition, even when it was using type embedding.

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