x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #817

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 12s
build (1.65.0, wasm32-unknown-unknown)	4m 11s
build (stable, thumbv7em-none-eabi)	29s
test (1.65.0)	12m 12s
build (stable, wasm32-unknown-unknown)	29s
test (stable)	7m 8s
minimal-versions / minimal-versions	5m 28s
	34m 9s

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