Use isoformat when producing json output in cloud-show-backup

This is an hotfix for support ticket 40034

This patch modifies the `to_json()` method of the BackupInfo class
substituting the `ctime` method from the datetime class with the
`isoformat` method, when printing `begin_time` and `end_time`
in json format.

The new format preserves all the relevant informations of the datetime
object, including the timezone.

References: 40034

Signed-off-by: Giulio Calacoci <giulio.calacoci@enterprisedb.com>

barman/infofile.py

@@ -714,9 +714,9 @@ def to_json(self):
         if data.get("tablespaces") is not None:
             data["tablespaces"] = [list(item) for item in data["tablespaces"]]
         if data.get("begin_time") is not None:
-            data["begin_time"] = data["begin_time"].ctime()
+            data["begin_time"] = data["begin_time"].isoformat()
         if data.get("end_time") is not None:
-            data["end_time"] = data["end_time"].ctime()
+            data["end_time"] = data["end_time"].isoformat()
         return data
 
     @classmethod