Feature request post failover command

Salut Dimitri,

Aim is to provide a parameter to execute a command (whatever command DBAs set) once a failover successfully happened

Use case is:
- sometimes after a failover we have to switch some settings (example switching Barman passive repository to active repository)
- another example is indirection (a.k.a switch-ip) if client does not support native high availability (e.g. they can't set two hosts on connection strings, example ODBC or embedded drivers configuration ), we could use this parameter (for example to reconfigure pgbouncer)

Because the only component which knows that a failover completely and succefullly happened is pg_autofailover.

- This command should be decorralated, if an error happens it simply should be list in the pg_autoctl log file.
- Most HA tools know provide this add-hoc parameter
Merci

Tiago