Switch from iText to OpenPDF module

[jodygarnett]

See https://github.com/geonetwork/core-geonetwork/pull/6343 for context from core-geonetwork project.

• Initial work is staged here https://github.com/geocat/mapfish-print/tree/2.2.x
• https://github.com/mapfish/mapfish-print-v2/pull/6