Mula metrics endpoint for current katalogus info

[underdarknl]

Currently, its hard to see if Mula has any up to date katalogus details for a given organization, and what those details are. I would propose a few metrics:

• The raw known katalogus set for a given organization.
• The time since the last refresh
• The refresh interval
• Possible the set of handled OOI_Types and their related scan levels s per the enabled boefjes
  Some of this data could be shown on the health endpoint too, as it helps with peeking inside the scheduler

[jpbruinsslot]

Relevant: #298