Add support for UNION DISTINCT BY NAME syntax

[alexander-beedie]

I was just enhancing Polars' SQL support for UNION ops and noticed the parser doesn't recognise UNION DISTINCT BY NAME; the presence of this (optional) additional keyword is supported by DuckDB (and some other dialects, such as U-SQL¹).

This PR adds a new DistinctByName entry to SetQuantifier to account for this, and streamlines the related test to avoid some copy/paste (while adding suitable coverage for the new quantifier) 👍

Example

• Setup some test data tables/frames:

  import polars as pl
  
  df1 = pl.DataFrame({
      "A": [1, 2, 3, 4, 5],
      "B": [5, 4, 3, 2, 1],
  })
  df2 = df1.select("B","A")

• Trying to execute this syntax using Polars (before this PR), which uses sqlparser-rs:

  pl.SQLContext( register_globals=True ).execute(
      "SELECT * FROM df1 UNION DISTINCT BY NAME SELECT * FROM df2"
  )
  # sql parser error: 
  #  Expected SELECT, VALUES, or a subquery in the query body, found: BY

• Execute same statement using DuckDB (to demonstrate that the syntax is valid):

  import duckdb
  duckdb.sql(
      "SELECT * FROM df1 UNION DISTINCT BY NAME SELECT * FROM df2"
  ).show()
  # ┌───────┬───────┐
  # │   A   │   B   │
  # │ int64 │ int64 │
  # ├───────┼───────┤
  # │     1 │     5 │
  # │     2 │     4 │
  # │     3 │     3 │
  # │     4 │     2 │
  # │     5 │     1 │
  # └───────┴───────┘

---

Thanks for all the fantastic work on this library :)

Footnotes

1. U-SQL "unions" reference: https://learn.microsoft.com/en-us/u-sql/statements-and-expressions/set-rowset/union-and-outer-union-expression ↩

[alamb]

Thank you very much for the contribution @alexander-beedie . The test cleanup was also very nice 👌

[alamb]

I took the liberty of merging up from main and fixing a clippy lint.