Add support for on-demand involvement

Fixes https://github.com/etkecc/baibot/issues/15

src/entity/interaction_context.rs

@@ -0,0 +1,13 @@
+use mxlink::ThreadInfo;
+
+use super::MessagePayload;
+
+pub struct InteractionContext {
+    pub thread_info: ThreadInfo,
+    pub trigger: InteractionTrigger,
+}
+
+pub struct InteractionTrigger {
+    pub is_mentioning_bot: bool,
+    pub payload: MessagePayload,
+}