Serialize SingleQubitCliffordGate

[ishmum123]

Closes #3980

I tried to mimic the PauliString Class for serialization -

Cirq/cirq-core/cirq/ops/pauli_string.py

Lines 175 to 185 in 4aeb719

	def _json_dict_(self) -> Dict[str, Any]:
	return {
	'cirq_type': self.__class__.__name__,
	# JSON requires mappings to have string keys.
	'qubit_pauli_map': list(self._qubit_pauli_map.items()),
	'coefficient': self.coefficient,
	}
	@classmethod
	def _from_json_dict_(cls, qubit_pauli_map, coefficient, **kwargs):
	return cls(qubit_pauli_map=dict(qubit_pauli_map), coefficient=coefficient)

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[ishmum123]

@googlebot I signed it!

[ishmum123]

@MichaelBroughton I am very sorry about the force pushes. My branch was showing a random error and incompatibility with the master

[dstrain115]

I wonder if it would be worth having a (parameterized) test in clifford_gate_test.py to run through all the single qubit clifford gates and do the round trip to json and from json and make sure they are equal. This might be a little bit higher confidence than having the single example in json_test_data, especially since that example has an empty inverse map.

Otherwise seems good to me.

[ishmum123]

Can you please check now?

[ishmum123]

@dstrain115 ping!

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[ishmum123]

@MichaelBroughton can you please check if this is okay?

[MichaelBroughton]

LGTM