Add PKCS#10 attributes to CSR serializer #1613

Job	Run time
Check semver compatibility	1m 2s
build-windows	15m 25s
build (macOS-latest, stable)	3m 25s
Build rustls-cert-gen as a standalone package	30s
build (ubuntu-latest, stable)	5m 40s
Validate external types appearing in public API	1m 33s
build (ubuntu-latest, beta)	5m 42s
build (ubuntu-latest, nightly)	5m 32s
Measure coverage	5m 48s
Format & clippy	5m 28s
build (ubuntu-latest, stable 7 months ago)	5m 55s
Documentation (stable)	57s
Documentation (nightly)	1m 7s
	58m 4s

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