Dissociate registrar API version from agent API version

[ansasaki]

Currently, the keylime agent uses its own API_VERSION to construct the accessed registrar API:

rust-keylime/keylime-agent/src/registrar_agent.rs

Lines 99 to 101 in c91fba3

	let addr = format!(
	"http://{remote_ip}:{registrar_port}/{API_VERSION}/agents/{agent_uuid}"
	);

This makes the agent API version tied with the registrar API version.

Evaluate if it makes sense to dissociate the versions, making it possible to advance the API versions independently.

[tuminoid]

+1.

We tried using the v0.2.6 agent (v2.2) versus a v2.1 registrar, and it doesn't find any endpoints. I did not found any config value that would help us here, and the API_VERSION is set as const. This is a problem as we're trying to deliver the agent images but do not control which registrar version target environment has, and delivering and supporting many agent images is less than optimal.