Use GoogleCZTargetGateset; add device gateset as additional gates in target gatesets (quantumlib#5765)

* GridDevice target gateset generation logic now matches all required gates for a particular gateset, rather than just the 2q gate.
* In target gatesets that support additional_gates, it's set to all gates in a device's gateset other than required gates in the target gateset, so that device gates are not decomposed during circuit transformation.

First commit is from quantumlib#5744

@tanujkhattar 
cc @dstrain115

Author: verult

cirq/transformers/target_gatesets/cz_gateset.py

@@ -56,7 +56,7 @@ def __init__(
             allow_partial_czs: If set, all powers of the form `cirq.CZ**t`, and not just
              `cirq.CZ`, are part of this gateset.
             additional_gates: Sequence of additional gates / gate families which should also
-              be "accepted" by this gateset. Defaults to `cirq.GlobalPhaseGate`.
+              be "accepted" by this gateset. This is empty by default.
         """
         super().__init__(
             ops.CZPowGate if allow_partial_czs else ops.CZ,

cirq/transformers/target_gatesets/sqrt_iswap_gateset.py

@@ -62,7 +62,7 @@ def __init__(
             use_sqrt_iswap_inv: If True, `cirq.SQRT_ISWAP_INV` is used as part of the gateset,
                 instead of `cirq.SQRT_ISWAP`.
             additional_gates: Sequence of additional gates / gate families which should also
-              be "accepted" by this gateset. Defaults to `cirq.GlobalPhaseGate`.
+              be "accepted" by this gateset. This is empty by default.
 
         Raises:
             ValueError: If `required_sqrt_iswap_count` is specified and is not 0, 1, 2, or 3.