feat(polars): support scalar subqueries for Polars backend

[lostmygithubaccount]

Is your feature request related to a problem?

in running TPC-H queries on the Polars backend with Ibis, 9/22 fail due to lack of scalar subquery support

What is the motivation behind your request?

mostly to have an issue to point to and discuss -- I'm not sure to what extent scalar subqueries are supported in Polars, but at least a very simple example works that fails for Ibis:

[ins] In [6]: df = t.to_polars()

[ins] In [7]: df.filter(df["bill_length_mm"] == df["bill_length_mm"].min())
Out[7]:
shape: (1, 8)
┌─────────┬────────┬────────────────┬───────────────┬───────────────────┬─────────────┬────────┬──────┐
│ species ┆ island ┆ bill_length_mm ┆ bill_depth_mm ┆ flipper_length_mm ┆ body_mass_g ┆ sex    ┆ year │
│ ---     ┆ ---    ┆ ---            ┆ ---           ┆ ---               ┆ ---         ┆ ---    ┆ ---  │
│ str     ┆ str    ┆ f64            ┆ f64           ┆ i64               ┆ i64         ┆ str    ┆ i64  │
╞═════════╪════════╪════════════════╪═══════════════╪═══════════════════╪═════════════╪════════╪══════╡
│ Adelie  ┆ Dream  ┆ 32.1           ┆ 15.5          ┆ 188               ┆ 3050        ┆ female ┆ 2009 │
└─────────┴────────┴────────────────┴───────────────┴───────────────────┴─────────────┴────────┴──────┘

Describe the solution you'd like

this would work:

[ins] In [1]: import ibis

[ins] In [2]: ibis.options.interactive = True

[ins] In [3]: ibis.set_backend("polars")

[ins] In [4]: t = ibis.examples.penguins.fetch()

[ins] In [5]: t.filter(t["bill_length_mm"] == t["bill_length_mm"].min())
Out[5]: 

[lots of traceback ommitted]

OperationNotDefinedError: No translation rule for <class 'ibis.expr.operations.subqueries.ScalarSubquery'>

What version of ibis are you running?

main, 9.1.0

What backend(s) are you using, if any?

Polars

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