x509-cert: introduce a `builder::AsyncBuilder` trait #2848

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	47s
build (1.71.0, wasm32-unknown-unknown)	44s
build (stable, thumbv7em-none-eabi)	42s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	37s
test (1.71.0)	3m 40s
test (stable)	3m 22s
	9m 52s

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