sha2: provide support for `spki::AlgorithmIdentifier` #1005

Job	Run time
cross (1.71.0, aarch64-unknown-linux-gnu, default)	45s
cross (1.71.0, powerpc-unknown-linux-gnu, default)	44s
cross (stable, aarch64-unknown-linux-gnu, default)	56s
cross (stable, powerpc-unknown-linux-gnu, default)	33s
minimal-versions / minimal-versions	44s
set-msrv / set-msrv	6s
test (1.71.0)	1m 22s
build (1.71.0, thumbv7em-none-eabi)	29s
test (stable)	45s
build (1.71.0, wasm32-unknown-unknown)	30s
build (stable, thumbv7em-none-eabi)	20s
build (stable, wasm32-unknown-unknown)	31s
	7m 45s

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