Add support to mix local and cloud execution of scenarios

[dgzlopes]

Probably there are better ways to tackle this problem. This is just one idea! Hope it doesn't sound too stupid :smile:

I would love to run some scenarios on the Cloud, and some on my machine at the same time, being both parts of the same test run.

This can be useful when:

• I've some scenarios that use extensions.
• I've some scenarios that have to run inside my network.

Example:

export let options = {
  scenarios: {
    example_scenario: {
      executor: 'shared-iterations',
      execution: local, // or something like that
      vus: 10,
      iterations: 200,
      maxDuration: '10s',
    },
    another_scenario: { ... }
  }
}

Then, when I use k6 cloud, run example_scenario locally, another_scenario in the Cloud, and see the results for the whole test run on the Cloud.