[SDFAB-844 - Part 2] Add SlicingConfig class

[ccascone]

This PR introduces a JSON schema for the slicing configuration as well as the SlicingConfig class for parsing the corresponding netcfg blob. The SlicingConfig class will be used by the NetcfgSlicingProvider to populate the SlicingManager stores.

TODO:

• Improve test coverage for parsing errors

Sample JSON config:

{
  "slices": {
    "0": {
      "name": "Default",
      "tcs": {
        "REAL_TIME": {
          "queueId": 1,
          "isSystemTc": true
        }
      }
    },
    "1": {
      "name": "P4-UPF",
      "tcs": {
        "CONTROL": {
          "queueId": 2,
          "maxRateBps": "2000000"
        },
        "REAL_TIME": {
          "queueId": 3,
          "maxRateBps": "50000000"
        },
        "ELASTIC": {
          "queueId": 4,
          "gminRateBps": "10000000"
        }
      }
    },
    "2": {
      "name": "BESS-UPF",
      "tcs": {
        "ELASTIC": {
          "queueId": 5
        }
      }
    }
  }
}

[pierventre]

Minor comments. Do you plan to address the TODO in this PR ?

[codecov]

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