PgFlow extensions migration version 3.

Adds attempts_count to the step_task_record composite type and replaces start_tasks with the upstream body plus that column, so a dispatched task carries the attempt it is on and PgFlow.Context.attempt can stop reporting a hardcoded 1.

Deploy the worker before applying this: PgFlow.Worker.TaskRow reads both the seven- and eight-column row, but a worker that predates it matches seven positionally and would fail every dispatch against the wider record. See priv/pgflow_helpers/sql/versions/v03/v03_up.sql for why the returned value is the stored count plus one.