Add KeyUsage support to CSR generation #1478

Job	Run time
Build rustls-cert-gen as a standalone package	35s
Validate external types appearing in public API	1m 30s
build-windows	9m 38s
build (macOS-latest, stable)	31s
Measure coverage	5m 54s
build (ubuntu-latest, stable)	1m 11s
Documentation (stable)	1m 8s
Format & clippy	5m 32s
Documentation (nightly)	1m 6s
build (ubuntu-latest, beta)	6m 12s
Check semver compatibility	1m 11s
build (ubuntu-latest, nightly)	5m 45s
build (ubuntu-latest, stable 7 months ago)	46s
	40m 59s

Provide feedback