feat: Support SQL "SELECT" with no tables, optimise registration of globals

[alexander-beedie]

Closes #16826.

• Allows SQL queries of the form SELECT 1 AS "one", 2.0 AS "two" (eg: project literals without reference to an existing table)¹.

Also:

• Improves/streamlines use of SQLContext with globals (cleaner/better handling of reserved SQL keywords).
• Removes a potential SQLContext footgun; can only automatically register non-Polars globals via pl.sql or SQLContext.execute_global now; this ensures that we only register objects referenced in an actual query.

Footnotes

1. PostgreSQL "SELECT" syntax: https://www.postgresql.org/docs/current/sql-select.html#:~:text=SELECT%20retrieves%20rows%20from%20zero%20or%20more%20tables. ↩

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.36%. Comparing base (c81ef69) to head (9a8ce01).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16836      +/-   ##
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- Coverage   81.38%   81.36%   -0.02%     
==========================================
  Files        1425     1425              
  Lines      187606   187612       +6     
  Branches     2697     2697              
==========================================
- Hits       152677   152649      -28     
- Misses      34434    34468      +34     
  Partials      495      495              

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