Add state to avoid exiting on wrong attestation

[sarroutbi]

This patch refactors the agent's state machine to make it more resilient
by preventing it from exiting on recoverable attestation or negotiation
errors. Previously, any failure during these stages would transition the
agent to a terminal Failed state.

This change ensures that the agent can recover from transient issues
(like network hiccups or temporary verifier unavailability) without
needing to be restarted. This is the list of the key changes:

• New AttestationFailed State: A new, non-terminal state
  AttestationFailed(error, context) has been introduced to specifically
  handle recoverable errors that occur during communication with the
  verifier.

• Retry Logic: The main run loop now handles the AttestationFailed
  state. When this state is reached, the agent will:

• Log the error: Wait for the default attestation interval.

• Transition: go back to the Negotiating state to retry the process.

• State Transitions: The handle_negotiation and attest methods have
  been updated. Instead of transitioning to the terminal Failed state on
  network errors or non-successful HTTP status codes, they now transition
  to the new AttestationFailed state, enabling the retry loop.

[sarroutbi]

/packit retest-failed

[codecov]

Codecov Report

❌ Patch coverage is 0% with 37 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.80%. Comparing base (df8e56e) to head (468e781).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/state_machine.rs	0.00%	37 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.80% <0.00%> (-0.14%)	⬇️
upstream-unit-tests	58.80% <0.00%> (-0.14%)	⬇️

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/state_machine.rs	18.43% <0.00%> (-2.59%)	⬇️

... and 5 files with indirect coverage changes

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[ansasaki]

LGTM!