Add fly_src to ProxyProtoOptions

[miguel-fly]

superfly/fly-proxy#2228 lets a TCP service opt in to a signed Fly-Src in its PROXY v2 header. This adds that option to ProxyProtoOptions, so customer machine services can set it:

# Example
{
  "port": 5432,
  "handlers": ["pg_tls", "proxy_proto"],
  "proxy_proto_options": {
    "version": "v2",
    "fly_src": true  # <--- I'm adding this
  }
}

MPG doesn't need this. Its endpoints are flaps virtual services, which use their own types.

The field is omitted when false, so existing configs serialize exactly as before.

Rollout

superfly/fly-proxy#2228 runs on every proxy, so this can deploy now, and a service can set fly_src as soon as it does.

Related

• https://github.com/superfly/mpg/issues/547