state: Add Available mapping for Assembly

- Utilize getResourceState and getResourceHealth utility function for
  Assembly Status.State and Status.Health
- Map `Available` to Redfish `UnavailableOffline`

Tested:
- Unit tests passed
- Redfish Service Validator passed
- Request expected
```
curl -k -v https://${bmc}/redfish/v1/Chassis/chassis/Assembly
```
```
{
  "@odata.id": "/redfish/v1/Chassis/chassis/Assembly",
  "@odata.type": "#Assembly.v1_6_0.Assembly",
  "Assemblies": [
    {
      "@odata.id": "/redfish/v1/Chassis/chassis/Assembly#/Assemblies/0",
      "@odata.type": "#Assembly.v1_6_0.AssemblyData",
      "Location": {
        "PartLocation": {
          "ServiceLabel": "Ufcs-N00-D0"
        }
      },
      "LocationIndicatorActive": false,
      "MemberId": "0",
      "Name": "base_op_panel_blyth",
      "Status": {
        "Health": "OK",
        "State": "Absent"
      }
    },
    {
      "@odata.id": "/redfish/v1/Chassis/chassis/Assembly#/Assemblies/1",
      "@odata.type": "#Assembly.v1_6_0.AssemblyData",
      "Location": {
        "PartLocation": {
          "ServiceLabel": "U78DA.N00.1234567-D1"
        }
      },
      "LocationIndicatorActive": false,
      "MemberId": "1",
      "Model": "6B86",
      "Name": "lcd_op_panel_hill",
      "PartNumber": "PN12345",
      "SerialNumber": "YL6B86010000",
      "SparePartNumber": "F191014",
      "Status": {
        "Health": "OK",
        "State": "UnavailableOffline"
      }
    }
  ],
  "Assemblies@odata.count": 2,
  "Id": "Assembly",
  "Name": "Assembly Collection"
}
```

Change-Id: If7f06b27dbdfa914a9db4480ba856d9dd25c8795
Signed-off-by: Justin Nguyen <justinnanguyen@gmail.com>