derive `Clone` for `Certificate` #1807

Job	Run time
build-windows	14m 21s
Validate external types appearing in public API	1m 38s
build (macOS-latest, stable)	3m 42s
build (ubuntu-latest, stable)	6m 55s
Build rustls-cert-gen as a standalone package	23s
build (ubuntu-latest, beta)	6m 53s
Measure coverage	6m 54s
Format & clippy	6m 28s
build (ubuntu-latest, nightly)	6m 54s
build (ubuntu-latest, stable 7 months ago)	6m 49s
Documentation (stable)	1m 20s
Check semver compatibility	58s
Documentation (nightly)	1m 14s
	1h 4m 29s

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