[breaking] gRPC: improved CompileRequest.export_binaries field definition

[cmaglie]

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• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
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• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

The optional gRPC field export_binaries now leverage the optional clause in latest version of protobuf 3 protocol.

What is the current behavior?

No change

What is the new behavior?

No change

Does this PR introduce a breaking change, and is titled accordingly?

Yes

Other information

related to #2565

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 69.56%. Comparing base (6faca0f) to head (fecb334).
Report is 1 commits behind head on master.

❗ Current head fecb334 differs from pull request most recent head 16e1ba6. Consider uploading reports for the commit 16e1ba6 to get more accurate results

Files	Patch %	Lines
commands/compile/compile.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2570      +/-   ##
==========================================
- Coverage   69.58%   69.56%   -0.02%     
==========================================
  Files         205      205              
  Lines       20267    20255      -12     
==========================================
- Hits        14102    14090      -12     
  Misses       5049     5049              
  Partials     1116     1116              

Flag	Coverage Δ
unit	69.56% <33.33%> (-0.02%)	⬇️

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