Add tooling_skip field option to preserve manually-maintained protobuf fields

[lucy66hw]

Description

Adds support for the [(tooling_skip) = true] option in protobuf files to mark fields that are manually maintained for gRPC and should not be marked as deprecated after postprocessing.

Test Plan

1, unit test.
2, Local test :
a) Add a [(tooling_skip) = true] option to protos/schemas/common.proto:
Map of fully-qualified field path -> typed value.
map<string, BinaryFieldValue> field_values = 4 [(tooling_skip) = true];
b) Run postprocessing processing:
c) Verified field_values remains in the output without [deprecated = true]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.41%. Comparing base (d74c385) to head (9e84400).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
+ Coverage   56.19%   56.41%   +0.21%     
==========================================
  Files          17       17              
  Lines        1397     1404       +7     
  Branches      375      382       +7     
==========================================
+ Hits          785      792       +7     
- Misses        528      611      +83     
+ Partials       84        1      -83     

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