Introduce `BlockGap` #2819

Job	Run time
set-image	10s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	12m 50s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	10m 40s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	31m 58s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	8m 46s
publish-benchmarks	0s
	1h 4m 24s

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