x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #204

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 6s
build (1.65.0, wasm32-unknown-unknown)	4m 8s
build (stable, thumbv7em-none-eabi)	28s
test (1.65.0)	6m 31s
test (stable)	32s
build (stable, wasm32-unknown-unknown)	30s
minimal-versions / minimal-versions	34s
	16m 49s

Provide feedback