Add fields for the Side effect retry feature

dev/restate/service/protocol.proto

@@ -47,6 +47,19 @@ message StartMessage {
 
   // If this invocation has a key associated (e.g. for objects and workflows), then this key is filled in. Empty otherwise.
   string key = 6;
+
+  // Retry count since the last stored entry.
+  //
+  // Please not this count might not be accurate, as it's not durably stored,
+  // thus it's susceptible to Restate's crashes/leader election changes.
+  uint32 retry_count_since_last_stored_entry = 7;
+
+  // Duration since the last stored entry.
+  // The time is set as duration since UNIX Epoch in milliseconds.
+  //
+  // Please note this time might not be accurate,
+  // and might change depending on which Restate replica executes the request.
+  uint64 duration_since_last_stored_entry = 8;
 }
 
 // Type: 0x0000 + 1
@@ -90,6 +103,11 @@ message ErrorMessage {
   optional string related_entry_name = 5;
   // Entry type.
   optional uint32 related_entry_type = 6;
+
+  // When to execute the next retry.
+  // If provided, it will override the default retry policy used by Restate's invoker.
+  // The time is set as duration since UNIX Epoch in milliseconds.
+  optional uint64 next_retry_at = 8;
 }
 
 // Type: 0x0000 + 4