mgr/telemetry: add command to list all channels

mgr/telemetry: add command to list all channels

List all channels, their current state, default, and description, with:

$ ceph telemetry channel ls

NAME      ENABLED    DEFAULT    DESC
basic     ON         ON         Share basic cluster information (size, version)
ident     OFF        OFF        Share a user-provided description and/or contact email for the cluster
crash     ON         ON         Share metadata about Ceph daemon crashes (version, stack straces, etc)
device    ON         ON         Share device health metrics (e.g., SMART data, minus potentially identifying info like serial numbers)
perf      ON         OFF        Share perf counter metrics summed across the whole cluster

Signed-off-by: Yaarit Hatuka <yaarit@redhat.com>