Fix boot time retrieval

[sarroutbi]

This patch correctly implements boot time retrieval for the Rust agent, replacing the temporary use of the current time with the actual system boot time.

• New boot_time.rs Module: A new module was created to contain the logic for getting the system boot time. It works by parsing the btime field from the /proc/stat file, which is the standard method on Linux.

• Updated Agent Payload: The agent no longer sends the current time (chrono::Utc::now()) as the boot_time. Instead, it now calls the new get_boot_time() function to provide the true boot time in its capabilities report.

• Error Handling: If reading the boot time fails, the agent now logs an error and defaults to the Unix epoch (1970-01-01), ensuring the program doesn't crash.

[ansasaki]

LGTM!

[codecov]

Codecov Report

❌ Patch coverage is 53.57143% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.99%. Comparing base (f17040d) to head (4dae70c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime/src/boot_time.rs	63.15%	7 Missing ⚠️
keylime-push-model-agent/src/struct_filler.rs	33.33%	6 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.99% <53.57%> (-0.01%)	⬇️
upstream-unit-tests	58.99% <53.57%> (-0.01%)	⬇️

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/struct_filler.rs	23.62% <33.33%> (+0.58%)	⬆️
keylime/src/boot_time.rs	63.15% <63.15%> (ø)

... and 4 files with indirect coverage changes

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