x509-cert: add `DirectoryString::BmpString` #1567

Job	Run time
build (1.85.0, wasm32-unknown-unknown)	33s
build (1.85.0, thumbv7em-none-eabi)	38s
test (stable)	28s
test (1.85.0)	34s
build (stable, thumbv7em-none-eabi)	43s
build (stable, wasm32-unknown-unknown)	27s
minimal-versions	-1s
	3m 22s

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