Feature Request: Expose new kamal-proxy features (HTTP/3, healthcheck port) in deploy.yml

kamal-proxy is shipping great features, but we can't use them via Kamal's deploy.yml yet.

Two current examples:

1. HTTP/3: Supported in proxy (PR #149) via --http3, but Kamal lacks a way (#146) to enable http3
2. Healthcheck port: Proxy supports custom ports, but Kamal config doesn't pass them. #1234 / #1667

Proposal: Could we either add explicit keys (http3, healthcheck.port) or allow generic args: or options: passthrough in the proxy block?

It would be awesome to use proxy updates on day one. Thanks!