Add Dux.exec/1 for raw DDL execution

[cigrainger]

Summary

• Add Dux.exec/1 for executing raw SQL that isn't a query — SET, INSTALL, LOAD, CREATE SECRET, CREATE TABLE, etc.
• These statements can't go through from_query/1 because it wraps SQL in SELECT * FROM (...)

Examples

Dux.exec("INSTALL httpfs; LOAD httpfs;")
Dux.exec("SET s3_region = 'us-east-1'")
Dux.exec("CREATE SECRET (TYPE s3, PROVIDER config, REGION 'us-east-1')")

Test plan

• Compiles with --warnings-as-errors
• Manual test in Livebook

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