Allow defining multiple keys

[loligans]

I have an attribute class that I want to use for defining multiple extended data keys, but the IJsonSchemaExtensionDataAttribute only supports a single key.

MyCustomAttribute : Attribute, IJsonSchemaExtensionDataAttribute
{
    public MyCustomAttribute(string test1value, string test2value) {
        Keys = new [] {"x-key1", "x-key2"}
        Values = new [] {test1value, test2value}
    }
}

The result is the schema generates 2 extended fields:
"x-key1": "test1value",
"x-key2": "test2value"

[loligans]

I created a PR that opens an API up to developers to define attributes that allow multiple kvp's