Adds ProcessConfigWHIPIngest struct to http/api/process.go and wires it
into ProcessConfig.Marshal() and ProcessConfig.Unmarshal() so that the
WHIPIngest.Enabled flag can be set via API and correctly propagates to
app.Config.WHIPIngest, which is checked by onWHIPProcessStart in
app/webrtc/whip_lifecycle.go.
Without this change, app.Config.WHIPIngest.Enabled was always false and
WHIP ingest could never activate regardless of what the UI sent.