sha2: provide support for `spki::AlgorithmIdentifier` #980

Job	Run time
minimal-versions / minimal-versions	42s
set-msrv / set-msrv	5s
test (1.71.0)	31s
build (1.71.0, thumbv7em-none-eabi)	25s
test (stable)	32s
build (1.71.0, wasm32-unknown-unknown)	19s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	23s
	3m 11s

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