feat(descriptor): add is_required method to FieldDescriptor

[Binlogo]

Add helper method to check if a field is required, which always returns false for proto3

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.59%. Comparing base (15bbdfe) to head (016b794).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   80.35%   80.59%   +0.23%     
==========================================
  Files          31       31              
  Lines        5295     5297       +2     
==========================================
+ Hits         4255     4269      +14     
+ Misses       1040     1028      -12     

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[andrewhickman]

Thanks!