cirq_google.GridDevice, minus gateset and gate durations (quantumlib#…

…5203)

Part of quantumlib#5050

* Implemented qubit set, qubit pairs, and op validation. Gateset and gate duration will be done in a follow-up.
* Added a `device_specification_validation` module shell, to be used by the QCS server side to validate the DeviceSpecification proto against things like: 
  * Qubit self-loops in qubit pairs
  * Qubit pairs contain qubits not in the valid qubit set.
  * Gate durations contains a gate which is missing in the gateset.

`__str__()` and `_repr_pretty_` are nearly all copied from `SerializableDevice`.

Part of quantumlib#5050

Also fixes quantumlib#5197

**Question:** Does this definition of device equality make sense, or should it contain device name as well? Implementing equality is convenient for json and repr tests but not sure if it make sense in general.

@dstrain115 @MichaelBroughton

cirq/devices/grid_device_metadata.py

@@ -189,7 +189,7 @@ def _from_json_dict_(
         return cls(
             qubit_pairs,
             gateset,
-            None if gate_durations is None else dict(gate_durations),
+            dict(gate_durations) if gate_durations is not None else None,
             all_qubits,
             compilation_target_gatesets,
         )