Simplify decomposition of controlled eigengates with global phase

[daxfohl]

Fixes #7238

1. Create a flag in DecomposeContext to configure extraction of terminal gates' global phases into distinct GlobalPhaseGates.
   a. This flag defaults to False for backward compatibility.
2. Instrument the terminal decomposition gates (X, Y, Z, CZ) to check this flag and extract global phase if it exists.
   a. Added some convenience methods in GlobalPhaseGate to help.
   b. Updated some unrelated existing code to use these convenience methods for clarity.
3. Update ControlledGate.decompose to attempt decompose the subgate before attempting the brute-force approach.
   a. This is what ultimately simplifies the decomposition result.
   b. Step 2 also allowed removal of the entire CZPowGate instanceof block there, since it was about extracting global phase.
4. Add unit tests for each change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (3578631) to head (0ce2c14).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7383   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files        1112     1112           
  Lines       97641    97707   +66     
=======================================
+ Hits        96359    96426   +67     
+ Misses       1282     1281    -1     

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