Add PKCS#10 attributes to CSR serializer #1625

Job	Run time
build-windows	13m 27s
Build rustls-cert-gen as a standalone package	26s
build (macOS-latest, stable)	4m 4s
Validate external types appearing in public API	1m 34s
build (ubuntu-latest, stable)	5m 48s
Measure coverage	6m 2s
build (ubuntu-latest, beta)	5m 54s
Format & clippy	5m 20s
Documentation (stable)	1m 23s
build (ubuntu-latest, nightly)	5m 36s
Documentation (nightly)	1m 7s
Check semver compatibility	1m 0s
build (ubuntu-latest, stable 7 months ago)	5m 51s
	57m 32s

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