Add a way to programmatically cancel runs when using the runner lib

[codyl-stripe]

Is your feature request related to a problem? Please describe.
I have a use case where I would like to run load indefinitely for most of the time, but occasionally want to programmatically cancel load runs.

This is currently not possible as https://pkg.go.dev/github.com/bojand/[email protected]/runner#Run blocks and there is no public access to configure a https://pkg.go.dev/github.com/bojand/[email protected]/runner#Requester which has the ability to stop a run.

Describe the solution you'd like
A clear and concise description of what you want to happen.
I would like to make newRequester to be public which would allow access to canceling runs.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

I do not believe there are any alternatives with the current public API