Revisit client_session_file/client_keylog_file when origin server side QUIC connection is supported

[masaori335]

There're features and settings to support session resumption (proxy.config.quic.client.session_file) and session keylog (proxy.config.quic.client.keylog_file) for ATS QUIC client implementations (traffic_quic cmd).
This might also useful when we support origin server side QUIC connection. But we need to revisit these to support multiple sessions which traffic_quic cmd doesn't need.

The concern was raised on #5231 (comment)

[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.

[maskit]

The setting for keylog was replaced with proxy.config.ssl.keylog_file by #8337, and it's available for all connections that uses TLS.