Added option to specify ClientCertificateContext in .NET 8+

[tyb-dev]

Proposed Changes

This allows clients to send the full intermediate chain during the TLS handshake instead of relying on OS stores or AIA fetches, addressing issue #1864. It improves interoperability when servers don’t have intermediates installed or when environments lack internet access.
Keep behavior unchanged for netstandard2.0; the new API is only compiled for net8.0.
Split Public API “Unshipped” files per TFM and wire them via AdditionalFiles to satisfy the PublicApiAnalyzer for both net8.0 and netstandard2.0.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
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Further Comments

N/A

[tyb-dev]

I haven't signed the CA yet. If there's interest in this change, I'm happy to sign it so this PR can proceed.

[michaelklishin]

@tyb-dev I definitely think there is interest in supporting this option (conditionally, on .NET 8+). Thank you!

Please email us as described in the contributor CLA repo and we will send you a Box Sign signature request.