Refactor: libcrmcommon: XML node for option metadata

Rather than building up an XML string manually, build up an XML node and
call a dump function.

This moves us closer to formatted output. We can adapt
pcmk__format_option_metadata() pretty closely to get a function for XML
output objects. We can write a new function for text output objects that
generates human-friendly text.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>