Introduce new gateway type: 'chat'

[nicoloboschi]

In many cases, the application declares two gateways for both producing and consuming messages at the end and beginning of the pipeline. This results in two connection opened to the gateway and hard configuration in the CLI.
It would be useful to have a simpler gateway that does both in only one connection and optimized for the chat-like communication.

Changes:

• Added a new type 'chat' to the gateway types

- id: chat
   type: chat
   authentication:
      provider: google
      configuration:
        clientId: "{{ secrets.google.client-id }}"
   chat-options:
      headers:
      - value-from-parameters: session-id
      - value-from-authentication: user-id
      answers-topic: output-topic
      user-parameter: subject

questions-topic: (required). Topic for producing messages. Typically the input topic of the pipeline
answers-topic: (required). Topic for reading messages. Typically the output topic of the pipeline
headers: (optional). When set, they become a required parameters to be passed to the gateway connection (like parameters in other gateway types). All the value-from-parameters are generated automatically by the CLI.

The producer and consumer implicitly uses session-parameter and user-parameter to append headers to the produced messages and use them as consumer filters.

The above chat example is functionally equivalent to:

- id: produce-input-auth-google
  type: produce
  topic: input-topic
  parameters:
    - session-id
  authentication:
      provider: google
      configuration:
        clientId: "{{ secrets.google.client-id }}"
    produce-options:
      headers:
        - key: subject
          value-from-authentication: subject
        - key: session-id
          value-from-parameters: session-id

  - id: consume-output-auth-google
    type: consume
    topic: output-topic
    parameters:
      - session-id
    authentication:
      provider: google
      configuration:
        clientId: "{{ secrets.google.client-id }}"
    consume-options:
      filters:
        headers:
          - key: subject
            value-from-authentication: subject
          - key: session-id
            value-from-parameters: session-id

• In the CLI you can connect to a chat gateway just by adding the id

langstream  gateway chat app chat -c TOKEN 

All the required parameters will be automatically generated by the CLI (UUID).