Add experimental complete-blob-backfill flag - #7751

Required conditions for merge:

• check-success=local-testnet-success
• check-success=test-suite-success

Check-runs and statuses of the embarked pull request #8077:

		license/cla Contributor License Agreement is not signed yet.	details
		checkpoint-sync-test-${{ matrix.network }}	details
		Check for 'skip-ci' label	details
		basic-simulator-ubuntu	details
		beacon-chain-tests	details
		cargo-sort	details
		cargo-udeps	details
		check-code	details
		check-msrv	details
		cli-check	details
		compile-with-beta-compiler	details
		debug-tests-ubuntu	details
		dockerfile-ubuntu	details
		doppelganger-protection-failure-test	details
		doppelganger-protection-success-test	details
		ef-tests-ubuntu	details
		execution-engine-integration-ubuntu	details
		fallback-simulator-ubuntu	details
		genesis-sync-test-electra-120s	details
		genesis-sync-test-electra-300s	details
		genesis-sync-test-fulu-120s	details
		genesis-sync-test-fulu-300s	details
		http-api-tests	details
		local-testnet-success	details
		lockbud	details
		network-tests	details
		op-pool-tests	details
		release-tests-ubuntu	details
		release-tests-windows	details
		run-local-testnet	details
		slasher-tests	details
		state-transition-vectors-ubuntu	details
		target-branch-check	details
		test-suite-success	details

❌ The pull request #7751 is part of a speculative checks batch that previously failed ❌