x509-cert: introduce a `builder::AsyncBuilder` trait (#1280) #2153

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	2m 14s
test (1.60.0)	2m 44s
build (1.60.0, wasm32-unknown-unknown)	2m 19s
test (stable)	28s
build (stable, thumbv7em-none-eabi)	11s
build (stable, wasm32-unknown-unknown)	12s
minimal-versions / minimal-versions	33s
	8m 41s

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