x509-cert: use `SubjectPublicKeyInfoOwned::from_key` #792

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	4m 7s
build (1.65.0, wasm32-unknown-unknown)	4m 59s
build (stable, thumbv7em-none-eabi)	29s
test (1.65.0)	4m 52s
build (stable, wasm32-unknown-unknown)	24s
test (stable)	26s
minimal-versions / minimal-versions	33s
	15m 50s

Provide feedback