x509-cert: introduce a `builder::AsyncBuilder` trait #1845

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 20s
build (1.65.0, wasm32-unknown-unknown)	2m 12s
build (stable, thumbv7em-none-eabi)	11s
test (1.65.0)	2m 24s
build (stable, wasm32-unknown-unknown)	13s
test (stable)	20s
minimal-versions / minimal-versions	22s
	8m 2s

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