push-model: refactor attestation logic into a state machine

[sergio-correia]

This commit refactors the main attestation logic into a state machine, simplifying the main function and making the agent's state

The state machine handles the agent's lifecycle, from registration to attestation.

[codecov]

Codecov Report

Attention: Patch coverage is 13.95349% with 111 lines in your changes missing coverage. Please review.

Project coverage is 57.77%. Comparing base (b3300fb) to head (089183b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/state_machine.rs	11.40%	101 Missing ⚠️
keylime-push-model-agent/src/main.rs	22.22%	7 Missing ⚠️
keylime-push-model-agent/src/attestation.rs	33.33%	2 Missing ⚠️
keylime-push-model-agent/src/url_selector.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	57.77% <13.95%> (-0.59%)	⬇️
upstream-unit-tests	57.77% <13.95%> (-0.59%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
keylime-push-model-agent/src/url_selector.rs	63.33% <66.66%> (-3.34%)	⬇️
keylime-push-model-agent/src/attestation.rs	40.97% <33.33%> (-0.17%)	⬇️
keylime-push-model-agent/src/main.rs	41.53% <22.22%> (-4.26%)	⬇️
keylime-push-model-agent/src/state_machine.rs	11.40% <11.40%> (ø)

... and 4 files with indirect coverage changes

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[ansasaki]

LGTM!