Extract ECAlgorithm from ECDsa and ECDiffieHellman

[vcsjones]

Extract a common ECAlgorithm base type from ECDsa and ECDiffieHellman.

Starting as draft to get feedback from CI, then will fill in XML doc comments that are missing.

Closes #61516

[ghost]

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

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Extract a common ECAlgorithm base type from ECDsa and ECDiffieHellman.

Starting as draft to get feedback from CI, then will fill in XML doc comments that are missing.

Closes #61516

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

[bartonjs]

The test that "failed" actually passed, then failed in infra post-testing:

[23:08:46] info: === TEST EXECUTION SUMMARY ===
[23:08:46] info: Total: 264, Errors: 0, Failed: 0, Skipped: 0, Time: 34.89062s