Basic test for `PjrtComputationClient`. (#4478)
* Basic test for `PjrtComputationClient`.
For now, the test does an end-to-end exercise of the client: it
transfers tensors to device, compiles a computation (R2 add),
and transfers the result back to host, ensuring correctness.
This works on CPU for now and is not yet executed during releases.
* Define static member in the cpp file rather than in header.
* Spell out types
