Add customizable postgres outbox

[jdon]

Summary

• add TransactionalOutbox trait to eventastic_postgres
• implement generic repository and transaction to delegate side-effect storage
• create eventastic_outbox_postgres crate with default table implementation
• update bank example to use new outbox crate

Testing

• cargo check

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https://chatgpt.com/codex/tasks/task_e_683f81e7a5888323ad5d98818029c57a