Add warning when converting global phase operator to OpenQASM 2.0

[kenya-sk]

OpenQASM 2.0 does not support global phase (https://github.com/Qiskit/qiskit/issues/7167#issuecomment-947821313). Therefore, the current implementation generated an error and terminated the program. Since the global phase does not affect the observation results, I have changed it to be supported as a warning and the program can be executed.

The following is how to handle the situation.

1. Determine if the operator passed to qasm() is an instance of ops.GlobalPhaseGate. If a Circuit is passed, it is deployed to each operator by existing implementations and a decision is made for each.
2. If it is an instance of ops.GlobalPhaseGate, generate a warning and return an empty string for compatibility with previous implementations.
3. Otherwise, the operation is the same as before.

---

fixes https://github.com/quantumlib/Cirq/issues/6457

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.76%. Comparing base (eef7c5c) to head (4281e86).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6476 +/- ## ======================================= Coverage 97.75% 97.76% ======================================= Files 1105 1105 Lines 94924 94936 +12 ======================================= + Hits 92793 92810 +17 + Misses 2131 2126 -5 ```

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[NoureldinYosri]

@kenya-sk thanks for taking care of this

[kenya-sk]

@NoureldinYosri Thank you for review. I have removed the changes that are not relevant to this issue.

[rht]

Thank you @kenya-sk @NoureldinYosri.