`polkadot-parachain`: add manual seal support #2088

Job	Run time
preflight / preflight	40s
preflight / ci-versions	55s
cargo-check-benches (master)	27m 26s
cargo-check-each-crate (1)	16m 47s
cargo-check-benches (current)	26m 57s
cargo-check-each-crate (2)	18m 3s
test-frame-ui	25m 49s
cargo-check-each-crate (3)	6m 39s
test-full-crypto-feature	2m 34s
cargo-check-each-crate (4)	15m 9s
cargo-check-each-crate (5)	23m 25s
test-node-metrics	8m 46s
cargo-check-each-crate (6)	12m 47s
cargo-check-each-crate (7)	47m 22s
cargo-check-all-crate-macos	10m 46s
test-frame-examples-compile-to-wasm	2m 58s
test-deterministic-wasm	6m 22s
check-tracing	2m 21s
check-metadata-hash	5m 54s
node-bench-regression-guard	13s
All test misc tests passed	1s
	4h 21m 54s

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