Adding FSM for upgrades

[ycombinator]

What does this PR do?

This PR adds a finite state machine to model the Elastic Agent upgrade process.

Why is it important?

To help with tracking the various steps in the upgrade process.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• ~I have made corresponding changes to the documentation
• ~I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool No user-facing changes in this PR.
• I have added an integration test or an E2E test

Related issues

• Relates Track and report upgrade details #3119

[elasticmachine]

💔 Build Failed

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• Start Time: 2023-10-03T01:50:13.530+0000

• Duration: 6 min 4 sec

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[elastic-sonarqube]

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[ycombinator]

It turns out it's not that useful to have an FSM modeling the upgrade states and their transitions for the purposes of upgrade tracking. An FSM may become more useful in the future if we actually use it to execute the various steps in the upgrade process. So, closing this PR unmerged for now.