Introduce validTime.mainDate

For many use-cases, the only the date component of validTime is displayed. Let's represent this information explicitly in CAAML.

    "validTime" : {
+     "mainDate" : "2024-12-09",
      "startTime" : "2024-12-08T16:00:00Z",
      "endTime" : "2024-12-09T16:00:00Z"
    },
    "tendency" : [ {
      "validTime" : {
+       "mainDate" : "2024-12-10",
        "startTime" : "2024-12-09T16:00:00Z",
        "endTime" : "2024-12-10T16:00:00Z"
      }
    } ],

https://avalanche.report/bulletin/2024-12-09