Pass run instance to task

[shalvah]

This PR adds a .run property to the task object, allowing the task to access information about the current run.

Some example use cases for this:

• Richer logs: automatically tagging logs emitted from the task with the run ID
• Richer error context: adding the run ID as a tag for better grouping/filtering
• Enriching the run metadata from within the task
• Persisting task information (for instance, marking an item as "failed" without crashing and interrupting the whole task)

It shouldn't be a breaking change, but I've wrapped it in a guard that first checks if the method exists.

Also, arguments are assigned afterwards, so if the task defines an attribute named run, it isn't overriden.

[olivier-thatch]

Any chance this could be reopened and considered for merging? We could certainly use this feature :)