x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #744

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 21s
build (1.65.0, wasm32-unknown-unknown)	4m 20s
test (1.65.0)	4m 35s
build (stable, thumbv7em-none-eabi)	28s
test (stable)	31s
build (stable, wasm32-unknown-unknown)	28s
minimal-versions / minimal-versions	42s
	15m 25s

Provide feedback