Snowflake Dialect pt. 6 - Union, Distinct and other improvements

[radeusgd]

Pull Request Description

• Part of Snowflake Dialect #9486
• Fixes Table.union, merge and distinct tests
• Replaces distinct_on in Context that was actually a Postgres specific addition leaking into the base with a more abstract Context_Extension mechanism.
  • This allows us to implement the Snowflake-specific DISTINCT using QUALIFY.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  TypeScript,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• Unit tests have been written where possible.