Migrate `pallet-im-online` to umbrella crate #47

Job	Run time
preflight / preflight	1m 21s
preflight / ci-versions	58s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	7m 10s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	5m 3s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	22m 27s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	3m 59s
publish-benchmarks	0s
	40m 58s

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