Add toStartOfDay datetime function

Author: asantoni

clickhouse_backend/models/functions/datetime.py

@@ -11,6 +11,7 @@
     "toStartOfTenMinutes",
     "toStartOfFifteenMinutes",
     "toStartOfHour",
+    "toStartOfDay",
     "toYYYYMM",
     "toYYYYMMDD",
     "toYYYYMMDDhhmmss",
@@ -80,6 +81,10 @@ class toStartOfHour(toStartOfMinute):
     pass
 
 
+class toStartOfDay(toStartOfMinute):
+    pass
+
+
 class toYearWeek(Func):
     output_field = fields.UInt32Field()