RollingInterval more extendable

[blowin]

Replace enum with class, and replace extension with method. This will allow you to more flexibly customize the spacing and format.

In my project, I need to use files with a specific name, but the library does not allow me to specify this. Therefore, I had to abandon this library and use a self-written solution.

I suggest the following solution:

public interface IRollingInterval
{
   string Format { get; }

   DateTime? CurrentCheckpoint(DateTime instant);

   DateTime? NextCheckpoint(DateTime instant);
}

With implementation: InfiniteRollingInterval, YearRollingInterval etc

This solution allow easy extend behaviour. To avoid backward compatibility issues, you can make a separate interface overload and mark the enum as obsolete.

[bartelink]

Realistically the current high level approach of defining rolling intervals is not something that's viable to change given the broad use of this sink

Closing as a duplicate of #243 and/or #298 - the most viable way for anything to change is to have a fork demonstrate something that works, take the learnings from that, map it to a feature request here, and have someone implement it