Allow defining custom converters matching fields with regexes

Some Confluence plugins define CQL fields with dynamic names.

For instance Metadata for Confluence. This can be noticed in the documentation of their metadata-overview macro.

The existing solution to use the component name / hints of the converter to determine which field it handles doesn't work in such dynamic context.

To make this work, we introduce a getHandledFields method returning a String or a Pattern that converters can implement. If they don't, or if they return null, we fall back to the component hints, keeping full backward compatibility with existing code and keeping the existing elegant and efficient way that uses the hints for fields with static names.