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