Add PKCS#10 attributes to CSR serializer #1627

Job	Run time
build (macOS-latest, stable)	10m 25s
Build rustls-cert-gen as a standalone package	24s
build (ubuntu-latest, stable)	5m 47s
build-windows	16m 7s
Validate external types appearing in public API	1m 35s
build (ubuntu-latest, beta)	5m 58s
build (ubuntu-latest, nightly)	5m 37s
Measure coverage	5m 53s
build (ubuntu-latest, stable 7 months ago)	5m 45s
Format & clippy	5m 17s
Documentation (stable)	56s
Documentation (nightly)	1m 6s
Check semver compatibility	1m 28s
	1h 6m 18s

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