add _phase_by_ method to ZZPowGate

[richrines1]

this was motivated by cirq.eject_z failing to commute through ZZPowGates, for example:

circuit = cirq.Circuit(
    cirq.Z(cirq.q(1)),
    cirq.ZZ(cirq.q(0), cirq.q(1)) ** 0.5,
    cirq.Z(cirq.q(1)),
)
print(cirq.eject_z(circuit))

output before:

0: ───────ZZ───────────
          │
1: ───Z───ZZ^0.5───Z───

output after adding _phase_by_:

0: ───ZZ───────
      │
1: ───ZZ^0.5───

(ZZPowGate is diagonal so this implementation is trivial)

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[viathor]

Two comments:

• Please sign the CLA.
• Optional: It'd be good to add a unit test (e.g. comparing the outcome of _phase_by_ to a circuit that conjugates ZZPowGate).

[richrines1]

* Please sign the CLA.

done.

* Optional: It'd be good to add a unit test (e.g. comparing the outcome of `_phase_by_` to a circuit that conjugates `ZZPowGate`).

added a simple test (which just checks that the output of cirq.phase_by() is now the same as the input). it looks like the result is already checked against the unitary obtained via explicit conjugation as part of assert_eigengate_implements_consistent_protocols - do you think it's worth adding another test like that here or is this sufficient?

[viathor]

Thank you!