Support percentiles as window functions

[carlineng]

Similar to #1555, but supporting the percentile calculations in a calculate: clause instead of an aggregate: clause. We should support both percentile_cont and percentile_disc, where cont interpolates between the two closest values, and disc choses the nearest.

See BQ documentation for intended behavior: https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#percentile_cont