Add wrapper struct to durable events and store trace context

[Aaron1011]

This should result in Sentry displaying a link from the task execution trace (where 'await_event' is called) back to the trace that performed the 'emit_event' call

Note that this is a breaking change, as we now wrap the user's payload in a struct when reading/writing to the database. Going forward, we'll be able to add new (optional) fields to this wrapper struct without breaking existing durable deployments

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Note

Introduces a structured event payload wrapper to support metadata (e.g., trace context) and links between traces.

• Adds DurableEventPayload { inner, metadata }; updates await_event/join to parse wrapper and return inner, and to link telemetry spans from metadata
• Client emit_event now wraps user payload, injects trace context into metadata, and sends wrapper to SQL
• SQL: emit_event validates payload keys (only inner, metadata); cleanup_task_terminal emits child completion events wrapped in the new format
• Types: AwaitEventResult.payload is now Option<DurableEventPayload>; re-exports include DurableEventPayload
• Errors: new EmitEventFailed variant for context-level emission failures
• Migration added to update functions and enforce payload structure; tests updated to use the wrapper

Breaking change: event payloads in e_* and checkpoints are now the wrapper object rather than raw JSON.

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[Aaron1011]

@virajmehta I opted to always store a json object that looks like {"inner": <obj>, "metadata": <object>}, including for the built-in durable events. This was much simpler than modifying all of the locking code to handle multiple columns.