Prepare cert tests for new signing algorithms

[bartonjs]

This is the test infrastructure half of #114357.

• CertificateAuthority gains algorithm agility, varying across RSA and EC-DSA deterministically.
• CertificateRequestChainTests stops using AsymmetricAlgorithm in advance of the new, non-AA types.
• CertificateRequestLoadTests ensures that Load+Create yields the same as new+Create for all supported signature generator types.
• PrivateKeyAssociationTests ensures the relationship of CopyWithPrivateKey, Get{Alg}PublicKey, Get{Alg}PrivateKey for all existing asymmetric algorithm types (even ECDH!)

[Copilot]

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

src/libraries/Common/tests/System/Security/Cryptography/X509Certificates/CertificateAuthority.cs:320

• Verify that the delegated CloneWithPrivateKey method robustly supports all key types (RSA, ECDsa, and DSA) and correctly converts from the generalized key type to the expected concrete instance.

return Common.CertificateAuthority.CloneWithPrivateKey(cert, key);

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @bartonjs, @vcsjones
See info in area-owners.md if you want to be subscribed.