Add KeyUsage support to CSR generation #1481

Job	Run time
Format & clippy	5m 14s
Check semver compatibility	47s
build (macOS-latest, stable)	17s
Validate external types appearing in public API	1m 27s
Build rustls-cert-gen as a standalone package	23s
build (ubuntu-latest, stable)	21s
build (ubuntu-latest, beta)	23s
build-windows	7m 26s
build (ubuntu-latest, nightly)	26s
Measure coverage	5m 38s
Documentation (stable)	59s
build (ubuntu-latest, stable 7 months ago)	30s
Documentation (nightly)	1m 3s
	24m 54s

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