Feature/add api datapoints

[Jixabon]

This PR aims to satisfy the following from API Improvements

• Add /system.raspapVersion
• Add /system.usedDisk
• Add /ap.frequency_band
• Add /clients endpoint
  • datapoints
    -active_clients_amount
    • active_wireless_clients_amount
    • active_ethernet_clients_amount
    • active_clients
      • will have same data as the interface specific list but an additional data point of "connection_type" with value of "wireless", "ethernet" or "unknown"
  • keep the existing /clients/{interface} endpoint for ability to get a specific interface's data

[billz]

@Jixabon tested each of the updated + new endpoints. all checks out. nicely done