move common metric server management part to own module

move common metric server management part to own module

For now it only handles the plugin registration and the two recently
integrated helpers.
But, this is a prepartation to move the external metrics server
update mechanic from a stateless always-newly-connect-send-disconnect
to a statefull transaction based mechanis; see later patches

keep the PVE::Status::Plugin use in pvestatd, as we read the cfs
hosted status.cfg there, and the parser is defined by the common
status plugin base module.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>