* eth/downloader: implement beacon sync

* eth/downloader: fix a crash if the beacon chain is reduced in length

* eth/downloader: fix beacon sync start/stop thrashing data race

* eth/downloader: use a non-nil pivot even in degenerate sync requests

* eth/downloader: don't touch internal state on beacon Head retrieval

* eth/downloader: fix spelling mistakes

* eth/downloader: fix some typos

* eth: integrate legacy/beacon sync switchover and UX

* eth: handle UX wise being stuck on post-merge TTD

* core, eth: integrate the beacon client with the beacon sync

* eth/catalyst: make some warning messages nicer

* eth/downloader: remove Ethereum 1&2 notions in favor of merge

* core/beacon, eth: clean up engine API returns a bit

* eth/downloader: add skeleton extension tests

* eth/catalyst: keep non-kiln spec, handle mining on ttd

* eth/downloader: add beacon header retrieval tests

* eth: fixed spelling, commented failing tests out

* eth/downloader: review fixes

* eth/downloader: drop peers failing to deliver beacon headers

* core/rawdb: track beacon sync data in db inspect

* eth: fix review concerns

* internal/web3ext: nit
Co-authored-by: Marius van der Wijden <m.vanderwijden@live.de>