Add SubjectPublicKeyInfo type #1549

Job	Run time
build (macOS-latest, stable)	4m 52s
Build rustls-cert-gen as a standalone package	26s
Validate external types appearing in public API	1m 29s
build (ubuntu-latest, stable)	5m 29s
build-windows	13m 53s
build (ubuntu-latest, beta)	5m 28s
Measure coverage	5m 28s
build (ubuntu-latest, nightly)	5m 28s
Format & clippy	5m 26s
build (ubuntu-latest, stable 7 months ago)	5m 41s
Documentation (stable)	1m 2s
Documentation (nightly)	1m 2s
Check semver compatibility	49s
	56m 33s

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