[BREAKING] Add protoc `oneof` clause where appropriate.

[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?

Add protoc oneof clause where appropriate.

What is the current behavior?

No change

What is the new behavior?

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

Yes.

Other information

Fix #2593

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.95652% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 70.26%. Comparing base (8b78c05) to head (8290196).

Files	Patch %	Lines
commands/service_debug_run.go	0.00%	12 Missing :warning:
commands/service_debug.go	0.00%	5 Missing :warning:
commands/service_monitor.go	33.33%	3 Missing and 1 partial :warning:
commands/service_library_uninstall.go	72.72%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2608 +/- ## ========================================== + Coverage 70.11% 70.26% +0.14% ========================================== Files 222 222 Lines 21124 21262 +138 ========================================== + Hits 14812 14939 +127 - Misses 5135 5147 +12 + Partials 1177 1176 -1 ``` | [Flag](https://app.codecov.io/gh/arduino/arduino-cli/pull/2608/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/2608/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arduino) | `70.26% <86.95%> (+0.14%)` | :arrow_up: | 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.

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