Add `CertificateSigningRequestDer` type #260

Job	Run time
Clippy	15s
Format	11s
msrv (ubuntu-20.04)	11s
Build + test (stable, ubuntu-20.04)	12s
msrv (macos-latest)	28s
Build + test (beta, ubuntu-20.04)	19s
Check semver compatibility	14s
msrv (windows-latest)	28s
Build + test (nightly, ubuntu-20.04)	15s
Build + test (macos-latest, stable)	42s
Build + test (windows-latest, stable)	39s
	3m 54s

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