Support percentile functions as aggregates

[carlineng]

We have gotten a lot of feature requests from users asking for percentile functions. We should support both percentile_cont, and percentile_disc. From the Snowflake documentation: "The function PERCENTILE_CONT interpolates between the two closest values, while the function PERCENTILE_DISC chooses the closest value rather than interpolating."

These are supported by DuckDB as aggregates: https://duckdb.org/docs/sql/aggregates#statistical-aggregates

Snowflake supports these as aggregates as well: https://docs.snowflake.com/en/sql-reference/functions/percentile_cont

BigQuery only supports these as window functions, which might make the implementation a bit tricky: https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#percentile_cont