Add serialization for Coupler Qids

[dstrain115]

• Adds serialization for coupler Qids to change them into a qubit string and back.

[codecov[bot]]

Codecov Report

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

Project coverage is 97.82%. Comparing base (c25678e) to head (1b3c6a7). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6682 +/- ## ========================================== - Coverage 97.82% 97.82% -0.01% ========================================== Files 1074 1074 Lines 92126 92161 +35 ========================================== + Hits 90124 90155 +31 - Misses 2002 2006 +4 ```

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