Add SQL exprs needed for custom offset window

[courtneyholcomb]

This includes a CASE expression, an integer expression, an arithmetic expression, and some updates to the add time expression & window function expressions. These are all needed to build the following SQL column:

CASE
  WHEN ds__martian_day__first_value__offset + INTERVAL (ds__day__row_number - 1) day <= ds__martian_day__last_value__offset
    THEN ds__martian_day__first_value__offset + INTERVAL (ds__day__row_number - 1) day
  ELSE ds__martian_day__last_value__offset
  END AS metric_time__day

Note that the first commit is moving the sql_exprs file into metricflow-semantics, which is needed for a commit further up the stack.