x509-cert: introduce a `builder::AsyncBuilder` trait #2083

Job	Run time
build (1.60.0, thumbv7em-none-eabi)	1m 35s
build (1.60.0, wasm32-unknown-unknown)	1m 47s
build (stable, thumbv7em-none-eabi)	16s
test (1.60.0)	1m 55s
build (stable, wasm32-unknown-unknown)	19s
test (stable)	30s
minimal-versions / minimal-versions	35s
	6m 57s

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