billing: Add billing format for PoolHitInfoMessage

Otherwise the Indexer is unable to parse these billing entries and then
appear as empty records in the json and yaml output.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7355/