feat: mark Function trait as Send (#12)

Allow to use Client in places that are also Send.

tests/v1_tool_test.rs

@@ -0,0 +1,7 @@
+use mistralai_client::v1::client::Client;
+
+trait _Trait: Send {}
+struct _Foo {
+    _dummy: Client,
+}
+impl _Trait for _Foo {}