feat: Add Certificate::from_der + Certificate::from_pem #1678

Job	Run time
Build rustls-cert-gen as a standalone package	26s
Validate external types appearing in public API	1m 31s
build (macOS-latest, stable)	3m 46s
build-windows	14m 3s
build (ubuntu-latest, stable)	6m 1s
Measure coverage	5m 47s
build (ubuntu-latest, beta)	6m 5s
build (ubuntu-latest, nightly)	6m 6s
Format & clippy	10s
build (ubuntu-latest, stable 7 months ago)	6m 7s
Documentation (stable)	54s
Check semver compatibility	57s
Documentation (nightly)	1m 1s
	52m 54s

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