push-model: implement continuous attestation with configurable intervals

[sergio-correia]

After the first successful attestation, the agent now waits for a
configurable interval before repeating the attestation process by
returning to the Negotiating state, creating a continuous attestation
loop.

The interval between the attestations is currently fixed, but in the
future, the verifier will provide this information in its response to
the attestation, so we can parse it from there and use it instead.

Currently, the interval between sending the measurements is defined
as 60s, but can be configured with the --attestation-interval-seconds
switch.

[codecov]

Codecov Report

❌ Patch coverage is 22.72727% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.21%. Comparing base (d7003ab) to head (96ea82c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/state_machine.rs	7.69%	12 Missing ⚠️
keylime-push-model-agent/src/attestation.rs	42.85%	4 Missing ⚠️
keylime-push-model-agent/src/main.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.21% <22.72%> (-0.69%)	⬇️
upstream-unit-tests	58.21% <22.72%> (-0.69%)	⬇️

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/main.rs	41.33% <50.00%> (-0.56%)	⬇️
keylime-push-model-agent/src/attestation.rs	44.92% <42.85%> (-16.15%)	⬇️
keylime-push-model-agent/src/state_machine.rs	15.32% <7.69%> (-28.62%)	⬇️

... and 5 files with indirect coverage changes

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[sarroutbi]

Please, define const values to avoid "magic numbers":

measurement_interval_seconds: 60, -> measurement_interval_seconds: DEFAULT_MEASUREMENT_INTERVAL_SECONDS

[ansasaki]

LGTM! I tested the continuous attestation against current state of the Verifier from the keylime/keylime#1693 and it worked well in the happy execution path.