crimson: add pg subcommands support in CLI

crimson: add pg subcommands support in CLI

Crimson OSD was missing the PG admin/tell hooks that classic OSD exposes, and it
did not accept the legacy `rados_pg_command()` / `ceph pg <pgid> <cmd>` JSON form
(e.g. `{"prefix":"pg","pgid":"1.0","cmd":"query",...}`), so `ceph pg <pgid> query`
failed.

Adds a `pg` old-form wrapper hook that exists to advertise that exists
to advertise the classic `pgid` + `cmd` + optional `arg` signature. The
runtime dispatch rewrites this to the real subcommand.

This updates parse_cmd  to rewrite `prefix=pg` requests to the requested
subcommand and remap the generic `arg` field to the concrete parameter
names (`offset` for `list_unfound`, `mulcmd` for `mark_unfound_lost`)
so validation/parsing is unambiguous.

Fixes: https://tracker.ceph.com/issues/73266
Signed-off-by: Kautilya Tripathi <kautilya.tripathi@ibm.com>