`agentsdk` depends on `agent/proto` because it needs to get the version to dial.
Therefore, the conversion routines need to live in `agentsdk` so that we can convert to and from the Manifest.
I briefly considered refactoring the agent to only reference `proto.Manifest`, but decided against it because we might have multiple protocol versions in the future, its useful to have a protocol-independent data structure.
This PR updates the Agent API to use the appearance.Fetcher, which is set by entitlement code in Enterprise coderd.
This brings the agentapi into compliance with the Enterprise feature.
