x509-cert: introduce a `builder::AsyncBuilder` trait #2227

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	30s
test (1.71.0)	22s
build (1.71.0, wasm32-unknown-unknown)	17s
test (stable)	24s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	10s
minimal-versions / minimal-versions	23s
	2m 18s

Provide feedback