x509-cert: introduce a `builder::AsyncBuilder` trait #1664

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	2m 9s
build (1.65.0, wasm32-unknown-unknown)	2m 12s
build (stable, thumbv7em-none-eabi)	19s
build (stable, wasm32-unknown-unknown)	10s
test (1.65.0)	2m 21s
test (stable)	9s
minimal-versions / minimal-versions	28s
	7m 48s

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