Add PKCS#10 attributes to CSR serializer #1614

Job	Run time
Validate external types appearing in public API	1m 33s
Check semver compatibility	52s
Build rustls-cert-gen as a standalone package	24s
build (macOS-latest, stable)	3m 35s
build-windows	13m 35s
Measure coverage	5m 55s
build (ubuntu-latest, stable)	5m 48s
Format & clippy	5m 23s
build (ubuntu-latest, beta)	5m 37s
build (ubuntu-latest, nightly)	5m 36s
Documentation (stable)	57s
build (ubuntu-latest, stable 7 months ago)	5m 50s
Documentation (nightly)	1m 2s
	56m 7s

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