QUIC will need ssl_client_cert_callback implementation after PR #6699 has landed

[shinrich]

I do not see where to put the analogous callback in the QUIC code, so filing this issue for others to remember and add it in.

[maskit]

The callback function, ssl_client_cert_callback should be set in quic_init_client_ssl_ctx().

I don't have strong opinion on where to put the callback function but I'd follow the same way as other XSupport classes and move most code currently in the callback function to the XSupport class for this.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. Marking it stale to flag it for further consideration by the community.