agent: Remove implementation of ToString

Fix clippy error:
```
direct implementation of `ToString`
```
by switching to implement Display instead

Signed-off-by: stevenhorsman <steven@uk.ibm.com>

src/agent/src/util.rs

@@ -181,13 +181,13 @@ mod tests {
         }
     }
 
-    impl ToString for BufWriter {
-        fn to_string(&self) -> String {
+    impl std::fmt::Display for BufWriter {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
             let data_ref = self.data.clone();
             let output = data_ref.lock().unwrap();
             let s = (*output).clone();
 
-            String::from_utf8(s).unwrap()
+            write!(f, "{}", String::from_utf8(s).unwrap())
         }
     }