recovery: optional restore_command & allow custom command if needed

recovery: optional restore_command & allow custom command if needed

Right now the role assumes you always want to use barman-wal-restore
script as a restore command to recover WAL files at startup time of a
standby server.

This PR adds a new `primary.restore_command` option which lets you
override the command to use.

⚠️ Breaking change: the PR renames the existing
`primary.restore_directory` option to
`primary.restore_barman_directory` ⚠️ in order to give more context to
this option which will automatically use the `barman-wal-restore`
script as a restore command.

Finally if none of the two options specified above are specified in
the `primary:` object then the `restore_command` is left commented out
in the PG configuration (which is totally fine as it will try to
recover WALs from the primary server directly see
[documentation](https://www.postgresql.org/docs/12/warm-standby.html#STANDBY-SERVER-OPERATION))