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/conversation/matrix/entity.rs

@@ -1,3 +1,4 @@
+use chrono::{DateTime, Utc};
 use regex::Regex;
 
 use mxlink::matrix_sdk::ruma::OwnedUserId;
@@ -8,6 +9,7 @@ pub struct MatrixMessage {
     pub message_type: MatrixMessageType,
     pub message_text: String,
     pub mentioned_users: Vec<OwnedUserId>,
+    pub timestamp: DateTime<Utc>,
 }
 
 #[derive(Clone)]