[macOS] Use data keys on macOS for RSA, EC cryptography

[filipnavara]

Resurrects the remaining work from #38101 (cc @vcsjones). Two code paths in S.S.C.X509Certificates are changed to explicitly import keys though the legacy CSSM keys because they are the only ones that reliably work with the macOS keychain APIs.

It also enables more of the RSA OAEP code paths through native code instead of using the managed RsaPaddingProcesser.

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Issue Details

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Test run for #51926 + reunification of the key import/export to use data keys on macOS. This should in theory be significantly faster but last CI run had one failing test and that needs to be investigated.

Author:	filipnavara
Assignees:	-
Labels:	area-System.Security
Milestone:	-

[filipnavara]

Hmm, just as expected it runs into the same trap as dotnet/corefx#27394. RSA PKCS #1 encryption/decryption on macOS 10.14 and older is broken for empty data (https://github.com/apple-opensource/Security/blob/e7e7d09ddf756349e64613a6890b0807ca06001f/OSX/sec/Security/SecRSAKey.c#L507). It's not a problem on recent iOS and macOS versions. It is a solvable problem by doing the de-padding on the managed side.

[filipnavara]

Side-note for reviewers: The GetPrivateKey method is called from one place which disposes the returned SafeHandle. In case we return a new handle here it will dispose it and then dispose the original RSA key. In the case of DSA below we reuse the internal handle and it gets disposed twice which is harmless.