Use the cord ctype for proto generation, where available.

This is natural as this field is often large and composed of
the concatenation of many small elements, which can lead to
significant performance improvements.

model/fn-execution/src/main/proto/org/apache/beam/model/fn_execution/v1/beam_fn_api.proto

@@ -663,7 +663,7 @@ message Elements {
     // (Optional) Represents a part of a logical byte stream. Elements within
     // the logical byte stream are encoded in the nested context and
     // concatenated together.
-    bytes data = 3;
+    bytes data = 3 [ctype = CORD];
 
     // (Optional) Set this bit to indicate the this is the last data block
     // for the given instruction and transform, ending the stream.