Add SubjectPublicKeyInfo type #1552

Job	Run time
Documentation (stable)	1m 1s
Documentation (nightly)	1m 2s
build-windows	10m 46s
Build rustls-cert-gen as a standalone package	26s
build (macOS-latest, stable)	3m 29s
Validate external types appearing in public API	1m 27s
build (ubuntu-latest, stable)	5m 51s
Measure coverage	5m 26s
build (ubuntu-latest, beta)	5m 27s
build (ubuntu-latest, nightly)	5m 18s
Format & clippy	5m 1s
build (ubuntu-latest, stable 7 months ago)	5m 39s
Check semver compatibility	1m 6s
	51m 59s

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