[feature] Feature request: Allow connect proxy to set SSE response headers

Please provide a mechanism for Connect Proxy to set and unset SSE response headers.

If a header name provided by Connect Proxy is among those normally managed by Centrifugo (e.g., CORS-related), the Connect Proxy's setting should take priority.

For example, {access-control-allow-origin: None} or {access-control-allow-origin: ""} would prevent access-control-allow-origin from appearing in the SSE response, even if Centrifugo would otherwise include it.