Add SubjectPublicKeyInfo type #585

Job	Run time
Validate external types appearing in public API	1m 30s
build (macOS-latest, stable)	3m 55s
Build rustls-cert-gen as a standalone package	47s
build (ubuntu-latest, stable)	5m 53s
build-windows	14m 32s
Measure coverage	5m 23s
build (ubuntu-latest, beta)	5m 29s
build (ubuntu-latest, nightly)	5m 26s
Format & clippy	5m 14s
Documentation (stable)	1m 11s
build (ubuntu-latest, stable 7 months ago)	5m 33s
Check semver compatibility	1m 4s
Documentation (nightly)	1m 3s
	57m 0s

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