Expose PKCS_RSA_PSS_SHA256 for CSR generation #1379

Job	Run time
Build rustls-cert-gen as a standalone package	1m 5s
Validate external types appearing in public API	1m 39s
build-windows	9m 41s
build (macOS-latest, stable)	48s
Measure coverage	8m 31s
Format & clippy	5m 21s
build (ubuntu-latest, stable)	47s
Documentation (stable)	2m 34s
build (ubuntu-latest, beta)	41s
Documentation (nightly)	1m 25s
Check semver compatibility	1m 20s
build (ubuntu-latest, nightly)	42s
build (ubuntu-latest, stable 7 months ago)	49s
	35m 23s

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