ippserver : Add support for attributes varying according to document format

[wifiprintguy]

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Some printers support different attributes depending on the document format being used. The Printer may limit the set of attributes reported by Get-Printer-Attributes when the operation request includes a "document-format" operation attribute , and a Client can use that to present the filtered set of choices once it has selected a document format. Similarly, the Printer may accept or reject a job creation operation request depending on whether a particular attribute is legitimate for a particular document format.

Currently there is no way to express such variances within a single Printer provided by ippserver. Separate Printers can be configured but those don't properly demonstrate this aspect of IPP.

Such an addition would require an extension to the ippserver's Print Service Configuration File format. One notion might be to adopt a convention from the venerable CUPS "ppdc" / "ppdcfile" system, which supported model variants that were delimited based on "curly bracket encapsulation" to indicate their scope.

[michaelrsweet]

I can't see using the ppdc format, which doesn't even support this sort of thing...

Putting in the Future milestone (not 1.0) for now, as I'm not sure how to best support this...