Define EvidenceHandling structures

[sarroutbi]

Related: #932

[codecov]

Codecov Report

Attention: Patch coverage is 82.53968% with 11 lines in your changes missing coverage. Please review.

Project coverage is 63.90%. Comparing base (1621e73) to head (39b15a1).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime/src/structures/evidence_handling.rs	82.53%	11 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	63.90% <82.53%> (+0.16%)	⬆️
upstream-unit-tests	63.90% <82.53%> (+0.16%)	⬆️

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Files with missing lines	Coverage Δ
keylime/src/structures/evidence_handling.rs	82.53% <82.53%> (ø)

... and 2 files with indirect coverage changes

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[ansasaki]

I think it is OK for now using a String as the error type, but in future it would be better to have the specific error types defined for evidence handling.