Allow to configure return type in JSON array parsing

[sbordet]

Target Jetty version(s)
9.4.x

Enhancement Description
Currently, JSON.parse("[1]") returns an Object[], while AsyncJSON returns List<Object>.

This is a bit inconsistent, although there is no standard, so that many different implementation return different types (e.g. Jackson vs Gson vs Jetty), so applications need to be aware of these differences and "sniff" the type before casting.

Would be great if at least within Jetty we can make this consistent, so for example provide AsyncJSON of an option such as parseArrayAsList=true|false so that it is consistent with the JSON class.

And perhaps the same to the JSON class.

For example, in CometD using the AsyncJSONTransport uses AsyncJSON and therefore arrays are parsed as lists, while WebSocketTransport uses JSON and so arrays are parsed as arrays.
Trying to deserialize that JSON into an object whose class has a setProperty(Object[] values), for example using JSONPojoConvertor, will work with one transport, but fail with the other, which is inconsistent.