Inject additional headers

[yassineselmi]

This PR offers the possibility to inject additional headers to the Langflow request. The main motivation behind this is to allow the user to send custom headers to bypass WAF or Security guardrails, as well as enforce CORS policies.

Example of usage:

<langflow-chat
    chat_input_field="input"
    chat_inputs='{"input":""}'
    host_url="langflow url"
    flow_id="your_flow_id"
    additional_headers='{"Access-Control-Allow-Origin": "myserver.com", "CF-Access-Client-Id": "1235678"}'
  ></langflow-chat>

[anovazzi1]

lgtm