Basic support for PDF/A-2 and PDF/A-3

[hvbargen]

Is your feature request related to a problem? Please describe.
PDF/A-3 is needed for ZUGFeRD (Factur-X) electronic invoices.
Besides, VeraPDF complains about PDF files generated with OpenPDF about CIDSet when I try to generate PDF/UA-1 (tagging works BTW).

Describe the solution you'd like
Constants in PDFWriter.java for PDF/A-2a, -2b, -2u and PDF/A-3a, -3b, 3u.
Similar to the existing constants for PDF/A-1 and with corresponding code adaptions where the existing PDF/A-1 constants are used
(PdfXConformance.java, XmpWriter.java, PdfXConformanceImp, TrueTypeFontUnicode.java).

Describe alternatives you've considered
I don't think there any alternative solutions, except maybe post-processing the PDF with Apache PDFBox ;-).

Your real name
Henning von Bargen

Additional context
I'm working on adding PDF/UA support to Eclipse BIRT.

[andreasrosdal]

Pull requests welcome. Thank you for reporting.