Returns the start of the week containing the date, shifted by an offset, if provided.
Start of the week is considered to be a Sunday.
date: The input date.
offset: An optional number of offset weeks from the input date (integer, default - 0).
A datetime representing the start of the week for the given date value, with the offset, if specified.
range offset from -1 to 1 step 1 | project weekStart = startofweek(datetime(2017-01-01 10:10:17), offset)