Migrate `pallet-authority-discovery` to umbrella crate #391

Job	Run time
preflight / preflight	43s
preflight / ci-versions	59s
build (polkadot-availability-recovery, availability-recovery-regression-bench)	7m 50s
build (polkadot-availability-distribution, availability-distribution-regression-bench)	5m 34s
build (polkadot-node-core-approval-voting, approval-voting-regression-bench)	23m 6s
build (polkadot-statement-distribution, statement-distribution-regression-bench)	4m 33s
publish-benchmarks	0s
	42m 45s

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