feat(proto): add EraseText RPC for InputText replace semantics

proto/driverpb/driver.proto

@@ -13,6 +13,9 @@ service Driver {
   rpc Tap(Point) returns (Empty);
   rpc TapSelector(Selector) returns (Empty);
   rpc InputText(Text) returns (Empty);
+  // EraseText deletes characters from the focused field so InputText can
+  // replace existing content instead of appending to it.
+  rpc EraseText(EraseTextRequest) returns (Empty);
   rpc Swipe(SwipeRequest) returns (Empty);
   rpc PressKey(PressKeyRequest) returns (Empty);
   rpc LongPress(Point) returns (Empty);
@@ -47,6 +50,10 @@ message Text {
   string value = 1;
 }
 
+message EraseTextRequest {
+  int32 character_count = 1;
+}
+
 message Selector {
   string value = 1;
 }