assigner: implements task assignment for scans and dummy mock test

[ashwiniag]

closes: #91

Introduces

• the task assigner logic in assigner.go, which is designed to distribute pending scans among available agents through round robin fashion. This assignment logic run independently as go routine and periodically at 1min interval
• adds assigner_test.go which just test interaction with HTTP endpoints and verifies that the assigner.go logic correctly interacts with endpoints. As follow:

• Basic Task Assignment
• No Agents Available
• No Scans Pending

• adds an endpoint for GET call in agentTasks for robust query

curl -X GET \
-H "Authorization: Bearer letsdoit" \
-H "Content-Type: application/json" \
"http://localhost:8000/api/v1/agents/tasks?scan_id=<scan-uuid>" | jq '.'

curl -X GET \
-H "Authorization: Bearer letsdoit" \
-H "Content-Type: application/json" \
"http://localhost:8000/api/v1/agents/tasks?agent_id=1" | jq '.'

Abstract Assigner Flow Logic

1. Fetch Pending Scans:
   Retrieve all scans with scans.Status=scan_pending that are waiting for assignment.

2. Fetch Available Agents:
   Identify agents with agents.Status=connected that are ready to process tasks.

3. Assign scans to agents via Round-Robin Assignment:
   Each scan is assigned to an agent in sequential order.
   Once the last agent is assigned, the logic loops back to the first agent.
   Used this thinking it ensures even distribution without overloading any single agent and retries any new batch of scans comes in with periodic execution.

4. Process Assignment
   Before assigning a scan, verify if it is already assigned to an agent in agentTasks to avoid duplication.

5. Assign Tasks:
   Create a task linking the scan and the assigned agent in in agentTasks.

6. Periodic Execution:
   The assigner runs at regular intervals to continuously check for new scans and available agents, ensuring no scan is left unprocessed.

Note:
This deosn't handle the logic of if scans have failed which are in scan.Status=error. Need to think on this