Add a new struct for representing agent key

MR: Add agent key struct to represent an agent (!2488 - merged)

Description

Currently, the agent id is represented as int64. As part of this epic, we are looking at introducing different type of agents

• agentk - Where each agentk is tied to a cluster_agent. This is the existing agent.
• agentw - Where each agentw is tied to a workspace. This is the new proposed agent.

Acceptance criteria

• Update the type used for representing the agent.
• Ensure changes are not done to the redis key. This will be done in a followup as part of Add new redis keys for agent key (#706 - closed)

Implementation plan

To be able to represent this correctly in the code, we need to migrate the agent id from an int64 to a struct like below

type AgentKey struct{
  ID   int64
  Type string
}