Set the maximum open connections limit in PG and MySQL target configs (#10558)

As the bulk/recursive delete will require multiple connections to open at an instance,
The default open connections limit will be reached which results in the following error

```FATAL:  sorry, too many clients already```

By setting the open connections to a reasonable value - `2`, We ensure that the max open connections
will not be exhausted and lie under bounds.

The queries are simple inserts/updates/deletes which is operational and sufficient with the
the maximum open connection limit is 2.

Fixes #10553

Allow user configuration for MaxOpenConnections

cmd/config/notify/help.go

@@ -340,6 +340,12 @@ var (
 			Optional:    true,
 			Type:        "sentence",
 		},
+		config.HelpKV{
+			Key:         target.PostgresMaxOpenConnections,
+			Description: "To set the maximum number of open connections to the database. The value is set to `2` by default.",
+			Optional:    true,
+			Type:        "number",
+		},
 	}
 
 	HelpMySQL = config.HelpKVS{
@@ -377,6 +383,12 @@ var (
 			Optional:    true,
 			Type:        "sentence",
 		},
+		config.HelpKV{
+			Key:         target.MySQLMaxOpenConnections,
+			Description: "To set the maximum number of open connections to the database. The value is set to `2` by default.",
+			Optional:    true,
+			Type:        "number",
+		},
 	}
 
 	HelpNATS = config.HelpKVS{