Add PKCS#10 attributes to CSR serializer #1595

Job	Run time
Validate external types appearing in public API	1m 30s
build (macOS-latest, stable)	3m 25s
build-windows	14m 40s
build (ubuntu-latest, stable)	5m 58s
Build rustls-cert-gen as a standalone package	27s
build (ubuntu-latest, beta)	6m 0s
build (ubuntu-latest, nightly)	5m 35s
Measure coverage	5m 57s
build (ubuntu-latest, stable 7 months ago)	6m 3s
Format & clippy	5m 39s
Documentation (stable)	56s
Documentation (nightly)	1m 7s
Check semver compatibility	56s
	58m 13s

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