x509-cert: introduce a `builder::AsyncBuilder` trait #1085

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 8s
build (1.65.0, wasm32-unknown-unknown)	2m 9s
test (1.65.0)	2m 32s
build (stable, thumbv7em-none-eabi)	13s
test (stable)	22s
build (stable, wasm32-unknown-unknown)	10s
minimal-versions / minimal-versions	33s
	8m 7s

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