Adding a Sync trait boundary to the ingest function's callback. (#3685)

The boundary is necessary to make the resulting future `Send`.
(Reference of `Sync` is `Send`).

`Send` is needed to tokio::task:spawn futures.

quickwit/quickwit-rest-client/src/rest_client.rs

@@ -255,7 +255,7 @@ impl QuickwitClient {
         index_id: &str,
         ingest_source: IngestSource,
         batch_size_limit_opt: Option<usize>,
-        on_ingest_event: Option<&dyn Fn(IngestEvent)>,
+        on_ingest_event: Option<&(dyn Fn(IngestEvent) + Sync)>,
         last_block_commit: CommitType,
     ) -> Result<(), Error> {
         let ingest_path = format!("{index_id}/ingest");