feat: Add default_value method

[andrewhickman]

Add default_value method to FieldDescriptor, ExtensionDescriptor and Value for better visibility

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.77%. Comparing base (6d9e8cb) to head (16c8d81).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
prost-reflect/src/descriptor/api.rs	93.10%	2 Missing ⚠️
prost-reflect/src/dynamic/mod.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   79.69%   79.77%   +0.08%     
==========================================
  Files          31       31              
  Lines        5250     5257       +7     
==========================================
+ Hits         4184     4194      +10     
+ Misses       1066     1063       -3     

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