Initial support for enum property validations (#29)

Reference: #18

If the OpenAPI specification declares the `enum` property and the attribute is determined to be configurable, adds the validator to the specification output.

Author: bflad

DESIGN.md

@@ -125,9 +125,9 @@ If not required, or if the field is in a different schema than the `Read` operat
 | Field (OAS)                                                                   | Field (Plugin Framework Schema)                                                                                                           |
 |-------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------|
 | [description](https://spec.openapis.org/oas/latest.html#rich-text-formatting) | [(Attribute).MarkdownDescription](https://developer.hashicorp.com/terraform/plugin/framework/handling-data/schemas#markdowndescription-1) |
+| [enum](https://json-schema.org/draft/2020-12/json-schema-validation.html#name-enum) | [(Attribute).Validators](https://developer.hashicorp.com/terraform/plugin/framework/validation) |
 | [format (password)](https://spec.openapis.org/oas/latest.html#data-types)     | [(StringAttribute).Sensitive](https://developer.hashicorp.com/terraform/plugin/framework/handling-data/schemas#sensitive)                 |
 
-
 ## Multi-type Support
 
 Generally, [multi-types](https://cswr.github.io/JsonSchema/spec/multiple_types/) are not supported by the generator as the Terraform Plugin Framework does not support multi-types. There is one specific scenario that is supported by the generator and that is any type that is combined with the `null` type, as any Plugin Framework attribute can hold a [null](https://developer.hashicorp.com/terraform/plugin/framework/handling-data/attributes#null) type.