Use pki_types to improve the interoperability with the rustls ecosystem #927

Job	Run time
build-windows	16m 48s
Build rustls-cert-gen as a standalone package	38s
Validate external types appearing in public API	1m 16s
build (macOS-latest, stable)	11m 17s
Clippy (--all-features)	5m 4s
build (ubuntu-latest, stable)	5m 47s
Measure coverage	5m 45s
Clippy (--no-default-features)	4m 47s
build (ubuntu-latest, beta)	5m 50s
Clippy (--no-default-features --features ring)	4m 44s
build (ubuntu-latest, nightly)	6m 6s
Documentation (stable)	44s
Clippy (--no-default-features --features aws_lc_rs)	4m 51s
build (ubuntu-latest, stable 7 months ago)	6m 22s
Format	14s
Documentation (beta)	42s
Clippy (--no-default-features --features aws_lc_rs,pem)	4m 51s
Documentation (nightly)	42s
Documentation (stable 7 months ago)	52s
Check semver compatibility	2m 51s
	1h 30m 11s

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