add legend information to coverages

[ricardogsilva]

This PR adds additional information to the coverage configuration detail endpoint (/api/v2/coverages7coverage-configurations/{cov-conf-if}) to expose information useful for generating a WMS legend graphic on the client side.

As seen below, the new legend property is a list of objects with colors and the respective value it represents on the underlying dataset.

{
  "url": "http://localhost:8877/api/v2/coverages/coverage-configurations/911e753a-1dc4-42e1-988b-3e514b885c9b",
  "id": "911e753a-1dc4-42e1-988b-3e514b885c9b",
  "name": "cdd_30yr_anomaly_seasonal_agree_model_ensemble",
  "legend": {
    "color_entries": [
      {
        "value": -40,
        "color": "#FF003c30"
      },
      {
        "value": -32,
        "color": "#FF01665e"
      },
      {
        "value": -24,
        "color": "#FF35978f"
      },
      {
        "value": -16,
        "color": "#FF80cdc1"
      },
      {
        "value": -8,
        "color": "#FFc7eae5"
      },
      {
        "value": 0,
        "color": "#FFf5f5f5"
      },
      {
        "value": 8,
        "color": "#FFf6e8c3"
      },
      {
        "value": 16,
        "color": "#FFdfc27d"
      },
      {
        "value": 24,
        "color": "#FFbf812d"
      },
      {
        "value": 32,
        "color": "#FF8c510a"
      },
      {
        "value": 40,
        "color": "#FF543005"
      }
    ]
  }
}

---

• fixes Add legend information to coverages #229

[ricardogsilva]

@francbartoli conflicts are now resolved