Add skip_default_fields option to text_format

[bouk]

As the name says, allows for text formatting a message with the default values included.

I've added a very simple 'Either' type to simplify the code, also using it for the serde implementation.

[codecov]

Codecov Report

Attention: Patch coverage is 85.29412% with 5 lines in your changes missing coverage. Please review.

Project coverage is 76.19%. Comparing base (8fd210d) to head (f54f011).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
prost-reflect/src/dynamic/serde/ser/mod.rs	80.00%	4 Missing ⚠️
prost-reflect/src/dynamic/text_format/format.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
+ Coverage   76.10%   76.19%   +0.09%     
==========================================
  Files          31       31              
  Lines        5473     5470       -3     
==========================================
+ Hits         4165     4168       +3     
+ Misses       1308     1302       -6     

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[andrewhickman]

Thanks for the PR, this looks like a useful flag to support. The changes look good, I'm happy to merge once the tests are passing

[bouk]

@andrewhickman I've pushed a commit but it's failing again with something that seems unrelated, the lint is also failing for code I didn't touch

[andrewhickman]

CI should be fixed now, thanks for the contribution!

This is now published in version 0.14.3