feat: Add option to run partition migration command outside transaction

[rpradal]

Having non atomic management command can help reduce lock contention.

In this PR a new atomic option has been added to the PostgresPartitioningConfig class. If set to false all changes will be done from outside a transaction. This option is set to True by default so it does not break retrocompatibility.

[kingbuzzman]

@rpradal What is the use case here? What pain point are you experiencing that motivated this change?