Add logging to the push attestation prototype

[ansasaki]

This adds log and pretty_env_logger as dependencies for the push attestation prototype.

This allows setting the log level through the RUST_LOG environment variable. For example, setting RUST_LOG=debug will make the debug messages to be logged.

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 63.61%. Comparing base (cb8070e) to head (efeca23).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime-push-model-agent/src/main.rs	58.33%	5 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	63.61% <58.33%> (+0.04%)	⬆️
upstream-unit-tests	63.61% <58.33%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
keylime-push-model-agent/src/main.rs	85.71% <58.33%> (-1.13%)	⬇️

... and 4 files with indirect coverage changes

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