feat: PERCENTILE_CONT function support

[MazterQyou]

This PR adds support for PERCENTILE_CONT(...) WITHIN GROUP (ORDER BY ...) parsing support. Related test is included.

[KSDaemon]

Do we need to support array versions of the percentile_cont?

percentile_cont ( fractions double precision[] ) WITHIN GROUP ( ORDER BY double precision ) → double precision[]
percentile_cont ( fractions double precision[] ) WITHIN GROUP ( ORDER BY interval ) → interval[]

[MazterQyou]

Do we need to support array versions of the percentile_cont?

percentile_cont ( fractions double precision[] ) WITHIN GROUP ( ORDER BY double precision ) → double precision[]
percentile_cont ( fractions double precision[] ) WITHIN GROUP ( ORDER BY interval ) → interval[]

Unlikely. We're only supporting parsing and planning of this, presumably to push it down, and many data warehouses don't seem to support that.

[coveralls]

Pull Request Test Coverage Report for Build 10704099334

Details

• 61 of 66 (92.42%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.529%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	27	29	93.1%
src/ast/mod.rs	10	13	76.92%

Totals
Change from base Build 8079365665:	0.03%
Covered Lines:	9063
Relevant Lines:	10123

---

💛 - Coveralls