feat: Add OneofDescriptor::is_synthetic

[andrewhickman]

Implements a better way to detect if a oneof is synthetic for #174

[codecov]

Codecov Report

❌ Patch coverage is 52.87356% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.77%. Comparing base (6d9e8cb) to head (988beb8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
prost-reflect/src/dynamic/message.rs	0.00%	12 Missing ⚠️
prost-reflect/src/descriptor/build/resolve.rs	25.00%	6 Missing ⚠️
prost-reflect/src/descriptor/build/options.rs	0.00%	4 Missing ⚠️
prost-reflect/src/descriptor/error.rs	75.00%	4 Missing ⚠️
prost-reflect/src/dynamic/text_format/format.rs	75.00%	4 Missing ⚠️
prost-reflect/src/dynamic/serde/de/kind.rs	50.00%	2 Missing ⚠️
prost-reflect/src/dynamic/serde/de/wkt.rs	33.33%	2 Missing ⚠️
prost-reflect/src/dynamic/text_format/parse/mod.rs	33.33%	2 Missing ⚠️
prost-reflect/src/descriptor/build/mod.rs	66.66%	1 Missing ⚠️
prost-reflect/src/dynamic/fields.rs	0.00%	1 Missing ⚠️
... and 3 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   79.69%   79.77%   +0.07%     
==========================================
  Files          31       31              
  Lines        5250     5255       +5     
==========================================
+ Hits         4184     4192       +8     
+ Misses       1066     1063       -3     

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