x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #907

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	5m 28s
test (1.65.0)	5m 23s
build (1.65.0, wasm32-unknown-unknown)	4m 12s
test (stable)	1m 35s
build (stable, thumbv7em-none-eabi)	26s
build (stable, wasm32-unknown-unknown)	25s
minimal-versions / minimal-versions	1m 25s
	18m 54s

Provide feedback