[proto] add RejectedTxIds to NotificationResponse (#89)

#### Type of change

- New feature

#### Description

Add a new RejectedTxIds message and field to NotificationResponse to
allow the notifier to report rejected transaction IDs with a reason
(e.g., resource exhausted, request too large). This enables graceful
rejection without terminating the stream.

#### Related issues

  - part of hyperledger/fabric-x-committer#405

Signed-off-by: Senthilnathan <cendhu@gmail.com>

Author: cendhu

api/committerpb/notify.pb.go

@@ -37,4 +37,11 @@ message NotificationRequest {
 message NotificationResponse {
     repeated TxStatus tx_status_events = 1; // List of transaction status events.
     repeated string timeout_tx_ids = 2;     // List of timeout events.
+    RejectedTxIds rejected_tx_ids = 3;      // List of rejected transaction IDs with a reason.
+}
+
+// RejectedTxIds contains a list of transaction IDs that were rejected along with the reason.
+message RejectedTxIds {
+    repeated string tx_ids = 1; // List of rejected transaction IDs.
+    string reason = 2;          // The reason for rejection.
 }