Add SubjectPublicKeyInfo type #1546

Job	Run time
Check semver compatibility	47s
Validate external types appearing in public API	1m 27s
Format & clippy	5m 0s
build-windows	15m 57s
Documentation (stable)	57s
Documentation (nightly)	59s
build (macOS-latest, stable)	4m 31s
build (ubuntu-latest, stable)	5m 20s
Build rustls-cert-gen as a standalone package	27s
build (ubuntu-latest, beta)	5m 51s
build (ubuntu-latest, nightly)	5m 25s
Measure coverage	5m 40s
build (ubuntu-latest, stable 7 months ago)	5m 45s
	58m 6s

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