feat: Add SQL support for TRUNCATE TABLE command

[alexander-beedie]

Ref: #7227.

• Adds SQL support for TRUNCATE <TABLE> xyz¹ commands. This is an optimal way to drop all rows from a table while preserving the table itself (so the schema doesn't change, but the table is empty).
• Adds test coverage for this and most of the other non-query directives (DROP, EXPLAIN, SHOW TABLES).

Example

import polars as pl

df = pl.DataFrame({
    "x": [1, 2, 3],
    "y": ["aaa", "bbb", "ccc"],
    "z": [date(2000, 12, 31), date(1978, 11, 15), date(2077, 10, 20)],
})

with pl.SQLContext(frame=df, eager_execution=True) as ctx:
    df1 = ctx.execute("TRUNCATE TABLE frame")
    df2 = ctx.execute("SELECT * FROM frame")
    
    df1
    # shape: (0, 3)
    # ┌─────┬─────┬──────┐
    # │ x   ┆ y   ┆ z    │
    # │ --- ┆ --- ┆ ---  │
    # │ i64 ┆ str ┆ date │
    # ╞═════╪═════╪══════╡
    # └─────┴─────┴──────┘
    
    df2
    # shape: (0, 3)
    # ┌─────┬─────┬──────┐
    # │ x   ┆ y   ┆ z    │
    # │ --- ┆ --- ┆ ---  │
    # │ i64 ┆ str ┆ date │
    # ╞═════╪═════╪══════╡
    # └─────┴─────┴──────┘

Footnotes

1. https://www.postgresql.org/docs/current/sql-truncate.html ↩

[codecov]

Codecov Report

Attention: Patch coverage is 76.47059% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.16%. Comparing base (cc6c642) to head (dd88b86).

Files	Patch %	Lines
crates/polars-sql/src/context.rs	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15513      +/-   ##
==========================================
+ Coverage   81.14%   81.16%   +0.01%     
==========================================
  Files        1362     1362              
  Lines      174989   175003      +14     
  Branches     2533     2533              
==========================================
+ Hits       142000   142034      +34     
+ Misses      32505    32485      -20     
  Partials      484      484              

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