Is it possible to use keywords (date units) in a function definition?

Hello,

I recently tried to write a wrapper function to calculate the difference
between two dates, mainly as a convenience. I'd essentially be emulating
EXTRACT(<dateunit> FROM date1 - date2), in various ways. I got a bit stuck
on allowing specification of the <dateunit>: is this possible in function
definitions? I'd like to be able to write something along the lines of:

CREATE OR REPLACE FUNCTION DATETIME_DIFF(end TIMESTAMP(3), start
TIMESTAMP(3), datepart UNIT) RETURNS DOUBLE PRECISION AS $$
BEGIN
RETURN EXTRACT(datepart FROM end - start);
END; $$
LANGUAGE PLPGSQL;

One option would be to treat datepart as a string, but this doesn't work
for my use case. (Background: I'm trying to refactor a bunch of SQL scripts
to work on Google BigQuery and PostgreSQL by writing PostgreSQL functions
to emulate BigQuery functions. Unfortunately BigQuery does not recognize
the third argument if it is a string (i.e. 'HOUR' does not work but HOUR
does)).

Any ideas? Is this even possible?

-Alistair