x509-cert: introduce a `builder::AsyncBuilder` trait #1573

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 33s
test (1.65.0)	1m 24s
build (1.65.0, wasm32-unknown-unknown)	1m 22s
test (stable)	17s
build (stable, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	17s
minimal-versions / minimal-versions	23s
	5m 34s

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