OpenApiSchema Required property

The type of `OpenApiSchema.Required` should be an `ISet<string>` instead of `IList<string>`

https://tools.ietf.org/html/draft-fge-json-schema-validation-00#section-5.4.3

The value of this keyword MUST be an array.  This array MUST have at
   least one element.  Elements of this array MUST be strings, and MUST
   be **unique**.