Add support for SUM window functions in slick-pg

[maximmold]

We would like to be able to use the sum window function documented here.

e.g. SELECT salary, sum(salary) OVER (ORDER BY salary) FROM empsalary;

[tminglei]

It's supported, you can check here for test cases.

[evbo]

See this unit test in particular, which shows windowing using avg: https://github.com/tminglei/slick-pg/blob/868f3666197ff3a0a1f233c2a9905feb483551f4/core/src/test/scala/com/github/tminglei/slickpg/PgWindowFuncCoreSuite.scala#L52