Add PreSpecified(Vec<u8>) option to KeyIdMethod. #685

Job	Run time
Validate external types appearing in public API	1m 5s
Format	20s
Clippy	4m 54s
Documentation (stable)	43s
Documentation (beta)	32s
build-windows	8m 25s
Documentation (nightly)	40s
build (macOS-latest, stable)	52s
Documentation (stable 7 months ago)	31s
Measure coverage	5m 22s
build (ubuntu-latest, stable)	32s
build (ubuntu-latest, beta)	29s
Check semver compatibility	26s
build (ubuntu-latest, nightly)	36s
build (ubuntu-latest, stable 7 months ago)	32s
	25m 59s

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