Track, return cost and usage metrics in chat api response

- Track input, output token usage and cost for interactions
  via chat api with openai, anthropic and google chat models

- Get usage metadata from OpenAI using stream_options
- Handle openai proxies that do not support passing usage in response

- Add new usage, end response  events returned by chat api.
  - This can be optionally consumed by clients at a later point
  - Update streaming clients to mark message as completed after new
    end response event, not after end llm response event
- Ensure usage data from final response generation step is included
  - Pass usage data after llm response complete. This allows gathering
    token usage and cost for the final response generation step across
    streaming and non-streaming modes

src/interface/obsidian/src/chat_view.ts

@@ -945,7 +945,7 @@ export class KhojChatView extends KhojPaneView {
             console.log("Started streaming", new Date());
         } else if (chunk.type === 'end_llm_response') {
             console.log("Stopped streaming", new Date());
-
+        } else if (chunk.type === 'end_response') {
             // Automatically respond with voice if the subscribed user has sent voice message
             if (this.chatMessageState.isVoice && this.setting.userInfo?.is_active)
                 this.textToSpeech(this.chatMessageState.rawResponse);