Initial SQL support

[billylanchantin]

Description

This PR experiments with initial SQL support by adding Explorer.DataFrame.sql/3 for making SQL queries against dataframes.

Example

iex> df = DF.new(a: [1, 2, 3], b: ["x", "y", "y"])
iex> DF.sql(df, "select a, b from df group by b order by b")
# #Explorer.DataFrame<
#   Polars[2 x 2]
#   a list[s64] [[1], [2, 3]]
#   b string ["x", "y"]
# >

Limitations

• Single table only for the moment, though Polars has multi-table functionality.
• No expressions.

[philss]

Awesome job! 🚀

I added some minor points :)

[josevalim]

@billylanchantin this is looking great! Is my understanding correct that we have two features in this pull request? One is to write SQL at the root level and another is to embed SQL inside mutate? Can I propose for us to add only the first one for now and then evolve it from there?

[billylanchantin]

@josevalim I'm good with that! DF.sql is more powerful and has actual documentation :)

Any particular hesitation with the embedded SQL part? Or are you just favoring smaller changes since this is experimental?

[josevalim]

Just favoring something smaller for now. It is always easier to add stuff than to remove them. :)