Add support for custom rollover strategy

DefaultRolloverStrategy has a very compact configuration, but as a trade-off has become a bit monolithic. I find it difficult to unit test and maintain.

Continuing in the spirit of the work done for LOG4J2-435, I would like to break out components for the different kinds of processing done by DefaultRolloverStrategy so that they can be individually configured and combined by users. Users would configure a CustomRolloverStrategy by specifying nested actions that define what needs to be done during a rollover.

The DefaultRolloverStrategy would still continue to work as current, but under the hood can delegate to these components.

At this stage the above is still just a concept. The description is to be updated when it becomes more clear if and how this would work.