Add slot based mechanism to execute pruners and feature alike

[gfukushima]

Currently teku has 3 different pruners (Block, blob and state) all relying on timers that trigger their execution at a frequency defined by the used. To avoid running those non-critical task during critical periods (like epoch transition) we could either:

1. Implement a validation to avoid pruning during these periods. This will likely end up being called / used in 3 different places.
2. Implement a mechanism that is slot based and only prune when we're in a "safer window" and will likely not affect the other critical tasks.

Other suggestion are accepted. @tbenr dumped some ideas here and here