x509-cert: make `RelativeDistinguishedName` field private (#1510) #3167

Job	Run time
minimal-versions	0s
build (1.72.0, thumbv7em-none-eabi)	29s
test (1.72.0)	1m 12s
build (1.72.0, wasm32-unknown-unknown)	36s
test (stable)	1m 8s
build (stable, thumbv7em-none-eabi)	33s
build (stable, wasm32-unknown-unknown)	29s
	4m 27s

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