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Datetime_Plus and TimestampAdd operators

There are essentially two ways to add an interval to a datetime vaue in

Call the timestampadd() function:
select {fn timestampadd(year, 1, hire_date)}...

Use datetime_plus interval arithmetic:
select hire_date + interval '1' year

Note that timestampadd's second argument does not need to be a literal.
Often it is a column expression. For datetime_plus literals are usually

I propose we create a new SqlOperator that can canonicalize both of these
inputs into one node. This lets us apply any transformations on this
canonical type regardless of what the original query was.

It takes in the following arguments:
1. A date/time/timestamp input
2. an interval input as an integer
3. a time unit for the input
4. a synthetic argument indicating the source form of the function call
(either datetime_plus or timestampadd).

The idea is that this canonical form is easy to get into for both types,
and provides methods to easily convert to either type. This would help with
unparsing in SqlDialects (you do not need to implement pushdown for both
types of inputs).