x509-cert: introduce a `builder::AsyncBuilder` trait #2150

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	2m 16s
build (1.60.0, wasm32-unknown-unknown)	2m 11s
build (stable, thumbv7em-none-eabi)	11s
build (stable, wasm32-unknown-unknown)	13s
test (1.60.0)	2m 41s
test (stable)	37s
minimal-versions / minimal-versions	29s
	8m 38s

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