Improve paras runtime `BenchBuilder` api (#4318)

* Improve paras runtime `BenchBuilder` api

* fix

* Improve doc comment

* Some doc improvemens'

* Use setters and no params for build

* +nightly-2021-10-29 fmt

* Clean up some comments

* Simplify set_code_upgrade

* Accept a slice for set_dispute_sessions

* Doc comments

* Spelling

* use impl AsRef

* Prepare for merge

* fmt

Co-authored-by: Bernhard Schuster <[email protected]>