Use bytes proto field type for string values of xla::PjRtValueType to bytes

`xla::PjRtValueType` is defined in C++, where its `std::string` value can
contain any string (not necessarily UTF-8). Protobuf verison 3 requires a
`string` field to contain UTF-8, so it is more suitable to use `bytes` to
express this value.

(Note that the string value of `xla::PjRtValueType` would be often consumed by
Python, where nanobind would convert `std::string` into Python `str` with UTF-8
decoding. However, this is what some users of `xla::PjRtValueType` choose to
do; this is not sufficient enough to constrain the string to be UTF-8 only in
C++ APIs.)

This is a preemptive change; there is no known problem of using a `string`
field previously.

PiperOrigin-RevId: 620315110

Author: hyeontaek

third_party/xla/xla/python/ifrt_proxy/common/types.proto

@@ -85,7 +85,7 @@ message Variant {
   }
 
   oneof value {
-    string string_value = 1;
+    bytes string_value = 1;
     sfixed64 int64_value = 2;
     Int64List int64_list = 3;
     float float_value = 4;