Add PKCS#10 attributes to CSR serializer #1617

Job	Run time
Validate external types appearing in public API	1m 32s
build-windows	15m 11s
Build rustls-cert-gen as a standalone package	24s
build (macOS-latest, stable)	4m 0s
Measure coverage	5m 56s
build (ubuntu-latest, stable)	5m 48s
Format & clippy	5m 29s
build (ubuntu-latest, beta)	5m 47s
Documentation (stable)	56s
Documentation (nightly)	1m 2s
build (ubuntu-latest, nightly)	5m 52s
Check semver compatibility	52s
build (ubuntu-latest, stable 7 months ago)	5m 45s
	58m 34s

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