x509-cert: introduce a `builder::AsyncBuilder` trait #2143

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	1m 32s
build (1.60.0, wasm32-unknown-unknown)	1m 33s
build (stable, thumbv7em-none-eabi)	16s
test (1.60.0)	1m 38s
build (stable, wasm32-unknown-unknown)	17s
test (stable)	19s
minimal-versions / minimal-versions	32s
	6m 7s

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