Add a contrib script for remote Barman recovery (#2931)

A contrib script, which can be used as a custom bootstrap method, or as a custom create replica method.

The script communicates with the pg-backup-api on the Barman node so Patroni is able to restore a Barman backup remotely.

The `--help` option of the script, along with the script docstring, should provide some context on how to use fill its parameters.

Patroni docs were updated accordingly to share examples about how to configure the script as a custom bootstrap method, or as a custom create replica method.

References: PAT-216.

docs/replica_bootstrap.rst

@@ -71,6 +71,22 @@ Makes the configured ``command`` to be called additionally with ``--arg1=value1
 
  .. note:: Bootstrap methods are neither chained, nor fallen-back to the default one in case the primary one fails
 
+As an example, you are able to bootstrap a fresh Patroni cluster from a Barman backup with a configuration like this:
+
+.. code:: YAML
+
+    bootstrap:
+        method: barman
+        barman:
+            keep_existing_recovery_conf: true
+            command: patroni_barman_recover
+            api-url: https://barman-host:7480
+            barman-server: my_server
+            ssh-command: ssh postgres@patroni-host
+
+.. note::
+    ``patroni_barman_recover`` requires that you have both Barman and ``pg-backup-api`` configured in the Barman host, so it can execute a remote ``barman recover`` through the backup API.
+    The above example uses a subset of the available parameters. You can get more information running ``patroni_barman_recover --help``.
 
 .. _custom_replica_creation:
 
@@ -125,6 +141,25 @@ example: pgbackrest
         basebackup:
             max-rate: '100M'
 
+example: Barman
+
+.. code:: YAML
+
+    postgresql:
+        create_replica_methods:
+            - barman
+            - basebackup
+        barman:
+            command: patroni_barman_recover
+            api-url: https://barman-host:7480
+            barman-server: my_server
+            ssh-command: ssh postgres@patroni-host
+        basebackup:
+            max-rate: '100M'
+
+.. note::
+    ``patroni_barman_recover`` requires that you have both Barman and ``pg-backup-api`` configured in the Barman host, so it can execute a remote ``barman recover`` through the backup API.
+    The above example uses a subset of the available parameters. You can get more information running ``patroni_barman_recover --help``.
 
 The ``create_replica_methods`` defines available replica creation methods and the order of executing them. Patroni will
 stop on the first one that returns 0. Each method should define a separate section in the configuration file, listing the command