x509-cert: introduce a `builder::AsyncBuilder` trait #2738

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	45s
build (1.71.0, wasm32-unknown-unknown)	37s
build (stable, thumbv7em-none-eabi)	30s
minimal-versions	0s
build (stable, wasm32-unknown-unknown)	27s
test (1.71.0)	1m 18s
test (stable)	1m 9s
	4m 46s

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