k6/execution: Export consolidated options

[codebien]

As a branch of #2259 issue and as a useful utility for having a more consistent behaviour for #796 and some other issues, we want to export a test.options object as part of the k6/execution API.

The common exported config object export const options = { ... } has some lack, it hasn't the capability to get accurate values within the VU's code like the following example:

export default function() {
  console.log(options.tags.mytag); // it prints undefined
}

The why is extensively explained by this comment:

The issue is that options get overwritten after they get merged with everything else (cli flags, env and so on). This overwriting though for some reason is really complicated and in this particular case options.tags is just set to its internal type that is a wrapper around a map, not just a map. As such it does not have properties such as mytag.

Proposal

import 'exec' from 'k6/execution';

export default function() {
  console.log(exec.test.options.tags.mytag);
}

• The exec code should export and convert in a nice read-only goja.Object the available consolidated and derived config from the lib.State.Options.
• A deprecation warning should be added when a set operation on the options object is executed. (Maybe also on getting?)