feat(chat): add the 'system' and 'tool' message roles (#10)

* add the 'system' and 'tool' message roles (see: https://docs.mistral.ai/capabilities/completion/ )

* docs(chat): add offical doc link in ChatMessageRole

* ci(github): listen to pull_request event in Test workflow

---------

Co-authored-by: Ivan Gabriele <ivan.gabriele@protonmail.com>

.github/workflows/test.yml

@@ -1,6 +1,8 @@
 name: Test
 
-on: push
+on:
+  pull_request:
+  push:
 
 jobs:
   test:

src/v1/chat.rs

@@ -30,12 +30,17 @@ impl ChatMessage {
     }
 }
 
+/// See the [Mistral AI API documentation](https://docs.mistral.ai/capabilities/completion/#chat-messages) for more information.
 #[derive(Clone, Debug, Eq, PartialEq, Deserialize, Serialize)]
 pub enum ChatMessageRole {
+    #[serde(rename = "system")]
+    System,
     #[serde(rename = "assistant")]
     Assistant,
     #[serde(rename = "user")]
     User,
+    #[serde(rename = "tool")]
+    Tool,
 }
 
 /// The format that the model must output.