Backing and collator protocol traces including para-id (#2620)

* improve backing/provisioner spans

* span for collation requests

* add para_id to unbacked candidate spans

* differentiate validation-construction and find-assignment in selection

* better find-assignment spans

* organize unbacked-candidate spans directly under job root

* Update node/core/provisioner/src/lib.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

Co-authored-by: Andronik Ordian <write@reusable.software>