rough-out new BackfillSync class structure

[vedant-asati]

Motivation
This goal of this pr is to restructure and rough-out new BackfillSync class structure.
Backfill issue: #7753

Description

• retained basic functional flow
• removed implementation specific code

WIP

[gemini-code-assist]

Summary of Changes

Hello @vedant-asati, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request initiates the restructuring of the backfill synchronization mechanism by introducing a new BackfillSync class. The primary goal is to establish a clear and robust architecture for future backfill operations, focusing on defining the core components and interactions without delving into the full implementation details at this stage.

Highlights

• New BackfillSync Class Structure: Introduces the foundational structure for a new BackfillSync class within packages/beacon-node/src/sync/backfill/v2/index.ts to address backfill issues.
• Core Components Defined: Defines essential types and interfaces such as BackfillSyncModules, BackfillSyncOpts, and BackFillSyncAnchor, laying out the dependencies and configuration for the backfill synchronization process.
• Initial Class Implementation: The BackfillSync class includes basic functional flow, peer connection/disconnection handling, and an init static method for proper initialization, while removing implementation-specific code as it's a rough-out.

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[gemini-code-assist]

Code Review

This pull request introduces a new, restructured BackfillSync class. The overall structure is a good starting point for the backfill sync logic. My review includes a couple of suggestions to improve maintainability and robustness: one regarding a naming inconsistency and another about the handling of the asynchronous sync process initialization.

[gemini-code-assist]

There's a typo in the type name BackFillSyncAnchor. For consistency with other names in this file (e.g., BackfillSyncModules, BackfillSyncOpts), it should be BackfillSyncAnchor.

Please correct this typo here and in its usages on lines 29 and 71.

Suggested change

	type BackFillSyncAnchor =
	| {
	anchorBlock: SignedBeaconBlock;
	anchorBlockRoot: Root;
	anchorSlot: Slot;
	lastBackSyncedBlock: BackfillBlock;
	}
	| {anchorBlock: null; anchorBlockRoot: Root; anchorSlot: null; lastBackSyncedBlock: BackfillBlock}
	| {anchorBlock: null; anchorBlockRoot: Root; anchorSlot: Slot; lastBackSyncedBlock: null};
	type BackfillSyncAnchor =
	| {
	anchorBlock: SignedBeaconBlock;
	anchorBlockRoot: Root;
	anchorSlot: Slot;
	lastBackSyncedBlock: BackfillBlock;
	}
	| {anchorBlock: null; anchorBlockRoot: Root; anchorSlot: null; lastBackSyncedBlock: BackfillBlock}
	| {anchorBlock: null; anchorBlockRoot: Root; anchorSlot: Slot; lastBackSyncedBlock: null};

[gemini-code-assist]

Starting an async operation in the constructor without awaiting it (a "fire-and-forget" promise) can make the system harder to reason about, especially for error handling and testing. Errors from the sync() process are only logged and not propagated to the creator of the BackfillSync instance.

Consider modifying the init method to return the sync() promise along with the instance. This would give the caller control over handling the completion or failure of the sync process.

For example:

// In BackfillSync class
public readonly syncPromise: Promise<void>;

constructor(...) {
  // ...
  this.syncPromise = this.sync().then(() => {
    this.logger.info("BackfillSync completed");
    this.close();
  }).catch((e) => {
    this.logger.error("BackfillSync processor error", e);
    this.status = BackfillSyncStatus.aborted;
    this.close();
    // Re-throw or handle to reject syncPromise
    throw e;
  });
}

static async init<T extends BackfillSync = BackfillSync>(
  opts: BackfillSyncOpts,
  modules: BackfillSyncModules
): Promise<{instance: T, done: Promise<void>}> {
  // ...
  const instance = new BackfillSync(opts, { ... }) as T;
  return {instance, done: instance.syncPromise};
}

This would be a more robust pattern.