x509-cert: introduce a `builder::AsyncBuilder` trait #1282

Job	Run time
build (1.70.0, thumbv7em-none-eabi)	25s
test (1.70.0)	1m 3s
test (stable)	50s
build (1.70.0, wasm32-unknown-unknown)	29s
build (stable, thumbv7em-none-eabi)	27s
build (stable, wasm32-unknown-unknown)	16s
minimal-versions / minimal-versions	1m 16s
	4m 46s

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