Open311 schema checking for comma separated data

e.g. status, service_request_id etc., which are documented
like "can be declared multiple times, comma delimited"

As the comma-separated data may be of specific (string-like)
types, we allow the type to be declared like:

    {
        type => '/open311/comma',
        contents => '/open311/status',
    }

We could also simply pass '//str' or similar.

We also accept a 'trim' parameter to remove leading/trailing
whitespace before checking value.