Introduce `BlockGap` #7813

Job	Run time
set-image	12s
check-runtime-migration (westend)	6m 41s
check-runtime-migration (rococo)	4m 33s
check-runtime-migration (asset-hub-westend)	4m 15s
check-runtime-migration (asset-hub-rococo)	5m 48s
check-runtime-migration (bridge-hub-westend)	3m 42s
check-runtime-migration (bridge-hub-rococo)	3m 43s
check-runtime-migration (contracts-rococo)	3m 48s
check-runtime-migration (collectives-westend)	3m 40s
check-runtime-migration (coretime-rococo)	3m 42s
All runtime migrations passed	1s
	40m 5s

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