Integrate exponential backoff to registration

[sarroutbi]

This commit integrates the ResilientClient into the registrar client, enabling automatic retries for agent registration. This enhances the agent's ability to handle transient network issues or temporary registrar unavailability.

• Configuration Propagation: A new RetryConfig struct is introduced and passed down from the agent's main configuration through agent_registration to the RegistrarClientBuilder.

• Resilient Registrar Client: The RegistrarClient is now equipped with an optional ResilientClient. If a retry configuration is provided, this client is used for both the initial version check and the final agent registration POST request.

• Enhanced ResilientClient: A new get_request method was added to the ResilientClient to handle requests that do not send a JSON body, such as the version check GET request.

[sarroutbi]

/packit retest-failed

[codecov]

Codecov Report

Attention: Patch coverage is 46.96970% with 35 lines in your changes missing coverage. Please review.

Project coverage is 58.74%. Comparing base (246883c) to head (e6711a0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
keylime/src/registrar_client.rs	43.18%	25 Missing ⚠️
keylime-push-model-agent/src/registration.rs	40.00%	9 Missing ⚠️
keylime/src/resilient_client.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
e2e-testsuite	58.74% <46.96%> (-0.14%)	⬇️
upstream-unit-tests	58.74% <46.96%> (-0.14%)	⬇️

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

Files with missing lines	Coverage Δ
keylime-agent/src/main.rs	15.64% <ø> (ø)
keylime-push-model-agent/src/attestation.rs	39.69% <100.00%> (ø)
keylime/src/agent_registration.rs	91.54% <100.00%> (+0.12%)	⬆️
keylime/src/resilient_client.rs	48.97% <75.00%> (+1.15%)	⬆️
keylime-push-model-agent/src/registration.rs	45.20% <40.00%> (-1.35%)	⬇️
keylime/src/registrar_client.rs	79.92% <43.18%> (-8.08%)	⬇️

... and 5 files with indirect coverage changes

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[ansasaki]

LGTM! I added some minor comments.

Just to register an idea: in future, I think we could improve this by making the RegistrarBuilder to return an impl and return different concrete structures depending on the Buider initialization instead of having the RegistrarClient to carry a ResilientClient inside it.