src/mon/PGMap.cc: init pool_availability

src/mon/PGMap.cc: init pool_availability

Added PoolAvailability Struct

Modified PGMap.cc to include a k,v map:
`pool_availability`.

The key being the `poolid` and value
is `PoolAvailability`

Init the function:
`PGMap::get_unavailable_pg_in_pool_map()`
to identify and aggregate all the PGs we
mark as `unavailable` as well as the pool
that associates with the unavailable PG.

Also, included `pool_availability`
to `PGMapDigest::dump()`.

Fixes: https://tracker.ceph.com/issues/67777
Signed-off-by: Kamoltat <ksirivad@redhat.com>