Add S.S.C.Pkcs and S.S.C.Cng netstandard2.1 ref and lib (dotnet/corefx#38439)

Add S.S.C.Pkcs and S.S.C.Cng netstandard2.1 ref and lib (dotnet/corefx#38439)

* Add S.S.C.Pkcs and S.S.C.Cng netstandard2.1 ref and lib

The netstandard2.1 ref is equivalent to the netcoreapp3.0 ref in these two
OOB packages.

Ref changes:

Cng:
* Adds DSACng and ECDiffieHellmanCng (and related types)

Pkcs:
* Adds RSA-OAEP option to CmsRecipient
* Adds custom private key option to CmsSigner
* Adds custom private key decrypt option to EnvelopedCms
* Adds Pkcs12Info and Pkcs12Builder (and associated types)
* Adds Pkcs8PrivateKeyInfo
* Adds RFC3161 timestamp types
* Adds AddCertificate/RemoveCertificate to SignedCms
* Adds AddUnsignedAttribute/RemoveUnsignedAttribute to SignerInfo
* Adds SignatureAlgorithm/GetSignature to SignerInfo
* Adds MatchesCertificate to SubjectIdentifier

Impl changes:

Cng:
* The netstandard2.1 impl is a generated PNSE, the real impl is inbox on Windows.

Pkcs:
* The impls are the same as the netcoreapp3.0 impls, but are required
because netstandard2.0 is a better match for netstandard2.1 than netcoreapp3.0 is.

* Use shared netstandard21 DefineConstant

* Add netstandard2.1 to ref AdditionalBuildConfigurations

Commit migrated from https://github.com/dotnet/corefx/commit/985334c90d6b181cd47cd42c4c958d9dd34ab90b