feat: add reorgs support + indexer restructuring

[PJColombo]

It resolves #19
It resolves #46
It resolves #49

It depends on the following API's PR

This PR contains the following refactoring:

• Renames slots_processor to synchroniser, which now encapsulates the slot checkpoint logic that was originally part of the indexer's logic. It also adds support for reverse syncing.
• Refactors the indexer's main logic by defining two parallel tasks, each with its own synchroniser instance:
  1. A historical syncing task that processes slots in reverse, from the last indexed one to either the genesis slot or a slot specified by the user.
  2. An event stream syncing task that listens to block head events and indexes them.

It also adds support for block reorgs.

[0xGabi]

I have conducted an initial review of the changes. I still need to cover some important sections on the synchronizer. Anyways I wanted to provide an initial round of feedback.

The refactor of the code has made it more robust with the addition of the synchronizer, which does a great job dividing the sync in two steps. Well done!

[0xGabi]

Fantastic job! I've noticed considerable improvements since my last review. I'm particularly impressed by how you've managed to split the synchronization into two separate processes, as we discussed. The addition of real-time sync significantly enhances the usability of Blobscan, allowing users to engage with it immediately without having to wait for the entire chain to synchronize.

Furthermore, the updates to the error messages are a substantial improvement. They now more effectively explain which dependency is causing an issue, which is super helpful.

I have very few additional suggestions to offer. Well done!