x509-cert: introduce a `builder::AsyncBuilder` trait #2827

Job	Run time
build (1.71.0, thumbv7em-none-eabi)	35s
build (1.71.0, wasm32-unknown-unknown)	33s
build (stable, thumbv7em-none-eabi)	21s
minimal-versions	0s
test (1.71.0)	38s
build (stable, wasm32-unknown-unknown)	19s
test (stable)	29s
	2m 55s

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