Follow up work for CMS

[PranavSenthilnathan]

PR #115310 is going to be merged soon but some smaller and/or out-of-scope items were deferred. This is to track that work:

• Disposing keys in CmsSignature more consistently
• Test NoSignature signing for unknown algos and throw exceptions if this path is not supported
• Avoid allocations in more places
  • Verify code path can use stackalloc/pool
  • Closures can be avoided with TState
• API Review - work will come up from the review including e.g. EditorBrowsable(Never)
• [blocked] Window compat tests - when PQC for Windows is released we need to test that netfx and our implementations match on some edge case scenarios where we don't have a strong opinion (NoSignature and CheckHash for example) or the spec is unclear (counter-signatures).

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