feat: add supabase_superuser role

[soedirgo]

Inherited by postgres and supabase_etl_admin. postgres retains
current privileges, while supabase_etl_admin gains the ability to
create event triggers

Part of PSQL-867

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/supabase-superuser

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