Change last_index to a uint64 (#1248)

This is a minor inconsistency with DataChunk's index field.

It remains backwards-compatible because uint32 and uint64 have the same wire protocol. https://protobuf.dev/programming-guides/encoding/

modal_proto/api.proto

@@ -719,7 +719,7 @@ message FunctionCallPutDataRequest {
 
 message FunctionCallGetDataRequest {
   string function_call_id = 1;
-  uint32 last_index = 2;
+  uint64 last_index = 2;
 }
 
 message Function {
@@ -1872,4 +1872,4 @@ service ModalClient {
 
 // Workspaces
   rpc WorkspaceNameLookup(google.protobuf.Empty) returns (WorkspaceNameLookupResponse);
-}
+}