Allow for prompt caching to work by using baibot_conversation_start_time_utc instead of baibot_now_utc

This patch introduces a new `baibot_conversation_start_time_utc`
variable which indicates the time the conversation got started.

Using `baibot_now_utc` is still possible, but given that the current
time is a moving target, its use is in conflict with prompt caching.

Because the new `baibot_conversation_start_time_utc` prompt variable
is a more reasonable default, we're now using it in all sample configs.

src/controller/chat_completion/mod.rs

@@ -492,6 +492,7 @@ async fn handle_stage_text_generation(
             .text_generation_model_id()
             .unwrap_or("unknown-model".to_owned()),
         chrono::Utc::now(),
+        conversation.start_time(),
     );
 
     let params = TextGenerationParams {