Add GridCoupler Qid to cirq_google

[dstrain115]

• This adds a Qid type to cirq_google that represents the coupler between two qubits.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.82%. Comparing base (bc4cd6d) to head (66b51b8). Report is 1 commits behind head on main.

Files	Patch %	Lines
cirq-google/cirq_google/devices/coupler.py	97.95%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6678 +/- ## ========================================== - Coverage 97.82% 97.82% -0.01% ========================================== Files 1072 1074 +2 Lines 92047 92118 +71 ========================================== + Hits 90046 90115 +69 - Misses 2001 2003 +2 ```

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