Optimize PlanFragment Serialization

[NikhilCollooru]

Do not send PlanFragment's jsonRepresentation, statsAndCosts to workers as they don't need it.
They are only for the coordinator UI purpose.

== NO RELEASE NOTE ==

[wenleix]

I did some performance profiling in production workload and it turns out serializing splits is dominating the TaskUpdateRequest serialization CPU. Serializing plan fragment is about 20%-35% of the time spent on serializing splits (i.e. TaskSource). Thus once #8964 get fixed, we no longer worry about time spent on serializing plan fragment :) .

This PR is still valuable since it reduce the bytes sent to worker from coordinator. Too large update has been demonstrated to cause troubles (see, e.g. #13360) and aligns with the Coordinator reliability efforts such as #13584 .

[wenleix]

Remove this comment as well

[wenleix]

For the commit subject, I would say "Optimize PlanFragment serialization".

"PlanFragment" is not an action, you cannot optimize "PlanFragment" :) .

[wenleix]

LGTM.