x509-cert: introduce a `builder::AsyncBuilder` trait #2022

Job	Run time
build (1.65.0, wasm32-unknown-unknown)	1m 46s
build (1.65.0, thumbv7em-none-eabi)	1m 37s
build (stable, wasm32-unknown-unknown)	23s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	5m 33s
build (stable, thumbv7em-none-eabi)	30s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	3m 44s
test (x86_64-unknown-linux-gnu, 1.65.0)	5m 33s
test (x86_64-unknown-linux-gnu, stable)	3m 43s
minimal-versions / minimal-versions	2m 24s
	25m 13s

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