Refactoring of the pluggable discovery client

[cmaglie]

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• The PR has no duplicates (please search among the Pull Requests
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• 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?

• Removed useless logic from the pluggable discovery client.
• Simplified handling of process-termination

What is the current behavior?

No change

What is the new behavior?

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

Other information

[cmaglie]

Moved to arduino/pluggable-discovery-protocol-handler#46