Add WillowGate to cirq_google serialization

[dstrain115]

• Adds WillowGate to serialization.
• Adds a special message for iswaplike gates so that we don't have to special case different values of FSimgate.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.66%. Comparing base (39306b4) to head (cec6a83).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7309      +/-   ##
==========================================
- Coverage   98.67%   98.66%   -0.01%     
==========================================
  Files        1108     1108              
  Lines       96220    96232      +12     
==========================================
+ Hits        94942    94952      +10     
- Misses       1278     1280       +2     

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