Add Ref.AsStringBytes to flatbuffers.flexbuffers Python API (#7713)

* Add Ref.AsStringBytes to flatbuffers.flexbuffers Python API

* Append Bytes to AsStringBytes return value

Co-authored-by: Jared Junyoung Lim <jaredlim@google.com>

python/flatbuffers/flexbuffers.py

@@ -727,6 +727,15 @@ class Ref:
   def IsString(self):
     return self._type is Type.STRING
 
+  @property
+  def AsStringBytes(self):
+    if self.IsString:
+      return String(self._Indirect(), self._byte_width).Bytes
+    elif self.IsKey:
+      return self.AsKeyBytes
+    else:
+      raise self._ConvertError(Type.STRING)
+
   @property
   def AsString(self):
     if self.IsString: