Add SubjectPublicKeyInfo type #1544

Job	Run time
Validate external types appearing in public API	1m 29s
build-windows	14m 37s
build (macOS-latest, stable)	3m 25s
Build rustls-cert-gen as a standalone package	32s
Measure coverage	5m 49s
Format & clippy	5m 17s
Documentation (stable)	1m 1s
build (ubuntu-latest, stable)	5m 32s
Documentation (nightly)	1m 0s
build (ubuntu-latest, beta)	5m 27s
build (ubuntu-latest, nightly)	5m 31s
Check semver compatibility	53s
build (ubuntu-latest, stable 7 months ago)	5m 33s
	56m 6s

Provide feedback