Move Agent identity information from the RegistrarClient structure to the new structure AgentIdentity

[ansasaki]

Introduce a new AgentIdentity structure that contains the Agent identity information, removing the corresponding data from the RegistrarClient structure.

[codecov]

Codecov Report

Attention: Patch coverage is 89.45148% with 25 lines in your changes missing coverage. Please review.

Project coverage is 69.40%. Comparing base (5050ee9) to head (4da24bb).

Files with missing lines	Patch %	Lines
keylime/src/registrar_client.rs	78.57%	24 Missing ⚠️
keylime/src/agent_identity.rs	99.09%	1 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	69.40% <89.45%> (-0.03%)	⬇️
upstream-unit-tests	69.40% <89.45%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
keylime/src/agent_registration.rs	93.05% <100.00%> (+0.40%)	⬆️
keylime/src/keylime_error.rs	62.96% <ø> (ø)
keylime/src/agent_identity.rs	99.09% <99.09%> (ø)
keylime/src/registrar_client.rs	86.66% <78.57%> (-7.36%)	⬇️

... and 5 files with indirect coverage changes

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[sarroutbi]

Minor suggestion for comments removal. Rest of changes LGTM