Add Support for Parsing Policy Templates and Expose in MarshallJSON

[independentid]

When parsing an AVP template such as:

permit(
    principal == ?principal,
    action in [avpTest::Action::"ReadAccount"],
    resource == ?resource
);

The parser will return an error: parser error: parse error at <input>:3:19 "principal": expected ident

As a work around, replacing ?resource and ?Principal with Template::"resource" etc seems to be a good enough workaround to convert to JSON format for now.