Carry the correlation id of the unit of work in an optional column.

[gustavofreze]

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Summary

The outbox can now record, next to each event, the correlation id of the unit of work that emitted it. A consumer
that receives the event can then log it under the same id as the request that caused it, instead of starting a new
trace at every hop.

• Columns::builder()->withCorrelationId(name:) enables an optional column. No column is written by default, so
  existing tables and layouts keep producing exactly the same statement and the same eight bindings.
• DoctrineOutboxRepository takes an optional correlationId reader, a Closure(): string called at each write. One
  repository serves every request of a long-lived container, because the value is read when the row is written, not
  when the repository is built.
• An empty string is stored as NULL, which is what a worker outside any request writes. A reader without the column,
  or the column without a reader, writes nothing or NULL, and never fails.
• The statement is now built from the column list, so the optional column goes last, after occurred_at.
• The README documents the column, the new parameter and the builder method.

Related issue

None.

Checklist

• Tests added or updated.
• Documentation updated when applicable.
• composer review passes.
• composer tests passes.