Blocks backfilling option - 1 Engine

[abi87]

Why this should be merged

Node that state synced may need to index all blocks and transactions below state summary height. This PR introduced the possibility to do so

How this works

Backfilling is split in the following PRs

• [X] Introduce new interface as well as empty implementation to unlock compilation. See https://github.com/ava-labs/avalanchego/pull/2343
• [X] Implement block retrieval in engine, via GetAncestors.
• [ ] Implement backfil handling in proposervm. See https://github.com/ava-labs/avalanchego/pull/2241
• [ ] Implement backfil in coreth. See https://github.com/ava-labs/avalanchego/pull/2255
• [ ] Experiment with bandwith aware peer tracker to speed up blocks download. See https://github.com/ava-labs/avalanchego/pull/2273

How this was tested

extra UTs + successful mainnet sync with https://github.com/ava-labs/avalanchego/pull/2273 code

[github-actions[bot]]

This PR has become stale because it has been open for 30 days with no activity. Adding the lifecycle/frozen label will cause this PR to ignore lifecycle events.