Annotate some APIs in System.Security.Cryptography.* as unsupported on iOS/tvOS

[MaximLipnin]

Part of #47910

Based on the System.Security.Cryptography.Xml.Tests library tests throwing PNSE on iOS/tvOS:

• System.Security.Cryptography.Xml.Tests.DSAKeyValueTest.GetXml
• System.Security.Cryptography.Xml.Tests.DSAKeyValueTest.GetXml_SameDsa
• System.Security.Cryptography.Xml.Tests.SignedXmlTest.AsymmetricDSASignature

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

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Issue Details

---

Part of #47910

Based on the System.Security.Cryptography.Xml.Tests library tests throwing PNSE on iOS/tvOS:

• System.Security.Cryptography.Xml.Tests.DSAKeyValueTest.GetXml
• System.Security.Cryptography.Xml.Tests.DSAKeyValueTest.GetXml_SameDsa
• System.Security.Cryptography.Xml.Tests.SignedXmlTest.AsymmetricDSASignature

Author:	MaximLipnin
Assignees:	-
Labels:	area-System.Security, new-api-needs-documentation
Milestone:	-

[filipnavara]

There is still unresolved (and untracked) issue how do we want to handle DSA.Create() and CryptoConfig on the iOS-like platforms. The current behavior is that it creates a DSA object that thows PNSE on basically all methods (key generation, Sign, Verify) which sounds quite useless. Alternative is to throw PNSE directly in the DSA.Create method and take DSA out of CryptoConfig. It's likely a bit cleaner and could save some code size. Depending on the preferred approach the PNSE/unavailable surface would likely be slightly different. I will file an issue and let someone decide on what is correct.

[filipnavara]

Superseded by #52978, feedback welcome.

[MaximLipnin]

Superseded by #52978, feedback welcome.

Thanks Filip, I'm closing it