Enable signing without private key #1545

Job	Run time
build (macOS-latest, stable)	3m 54s
build-windows	12m 24s
build (ubuntu-latest, stable)	5m 51s
Build rustls-cert-gen as a standalone package	32s
build (ubuntu-latest, beta)	5m 42s
Validate external types appearing in public API	1m 29s
build (ubuntu-latest, nightly)	5m 30s
build (ubuntu-latest, stable 7 months ago)	5m 54s
Measure coverage	5m 27s
Format & clippy	5m 24s
Documentation (stable)	1m 3s
Check semver compatibility	1m 1s
Documentation (nightly)	1m 4s
	55m 15s

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