feat: add num_attempts to merge insert result (#2795)

This pipes the num_attempts field from lance's merge insert result
through lancedb. This allows callers of merge_insert to get a better
idea of whether transaction conflicts are occurring.

docs/src/js/interfaces/MergeResult.md

@@ -8,6 +8,14 @@
 
 ## Properties
 
+### numAttempts
+
+```ts
+numAttempts: number;
+```
+
+***
+
 ### numDeletedRows
 
 ```ts