Create k6runner.Script and make probers aware of it

[nadiamoe]

Before this PR, MultiHTTP and Scripted Probers created k6 runners using only the raw script data ([]byte) as input to the runner. This have worked nicely, as until now runners only required two things to work: The raw script data, and a timeout that was encoded in the context for Runner.Run().

We have seen however a couple scenarios that make me think this is not enough for the near future:

• We want to decouple the context from Runner.Run(), a.k.a. the request timeout, from the check timeout that is communicated to the runners. This is needed to properly implement what k6runner: use check context for http request #715 workarounded.
• We want to make k6 runners aware of some check metadata, for troubleshooting. https://github.com/grafana/sm-k6-runner/pull/191 worked around this using a hash, but it's not ideal.
• We want to be able to implement different queues for browser and non-browser scripts (https://github.com/grafana/sm-k6-runner/issues/174). We tentatively want to do this by extracting script features (https://github.com/grafana/sm-k6-archiver/pull/22), which will eventually need to be sent to the runners.

For these three reasons, I think it is beneficial to create a richer type so that Probers can feed all these details to the k6 runner, which this PR does.

Please note that this PR is not feature complete, namely how timeouts are handled does not fully makes sense. This is because I tried to keep the patchset as small as possible for reviewability. How timeouts are handled is improved in #724.

[nadiamoe]

This is the core change of this PR: Replace the []byte argument in Runner.Run with the new Script type. All other changes are essentially adapting the rest of the codebase to this change.