Make time partition suffix customizable

[ckrybus]

An implementation attempt for #197

Usage example:

manager = PostgresPartitioningManager(
    [
        PostgresPartitioningConfig(
            model=PartitionedMonthlyModel,
            strategy=CustomPostgresCurrentTimePartitioningStrategy(
                size=PostgresTimePartitionSize(months=1),
                count=3,
                name_format="%Y_%m",     # <--- new functionality
            ),
        ),
    ]
)

The change is fairly clean and it works, but design-wise I'm not sure if it is the right approach. Another possibility would be to add is as PostgresTimePartitionSize parameter, because the new parameter is "coupled to a specific size" (e.g. using %Y_%m_%d together with PostgresTimePartitionSize(months=1) does not really make sense). That said, PostgresTimePartitionSize is not responsible for naming, so 🤷‍♂️

[Photonios]

Looks acceptable to me! The fact that you might shoot yourself in the foot by doing %Y_%m_%d with PostgresTimePartitionSize(months=1) is acceptable to me. I suppose you know what you're doing if you use this parameter.