[breaking] gRPC: improved CompileRequest.export_binaries field definition

[cmaglie]

Please check if the PR fulfills these requirements

See how to contribute

• [X] The PR has no duplicates (please search among the Pull Requests before creating one)
• [X] The PR follows our contributing guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)
• [X] UPGRADING.md has been updated with a migration guide (for breaking changes)
• [ ] 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[bot]]

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.

:exclamation: 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 :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/arduino/arduino-cli/pull/2570/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arduino) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/arduino/arduino-cli/pull/2570/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arduino) | `69.56% <33.33%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arduino#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.