Extra env/tags/exec options for executors

[na--]

Users should be able to inject environment variables and metric tags per-executor, as well as be able to specify a custom function to be executed instead of the default one. Something like this:

export let options = {
    execution: {
        someKey: {
            type: "constant-looping-vus",
            vus: 10,
            duration: "60s",

            // Inject/overwrite environment variable key-value pairs
            env: {
                blah: "whatever",
                key1: "value1",
                key2: "value2"
            },

            // Allow users to execute some other *exported* function besides the default one
            exec: "someExportedFunctionThatsNotTheDefault",

            // The ability to inject additional metric tags
            tags: { tag1: "value1", tag2: "value2" },
        },
    },
}

These things would allow us to relatively gracefully support multiple scenarios in the same script, a very frequently requested feature.

[na--]

My refactoring proposal in #1283 (comment), while apparently not very useful for solving that particular issue as I originally meant it to, still seems to me to be a good way both clean the current overly-complicated code and to implement the features I describe above.

The only missing piece from it is the validation of the exec option - we need to validate that someExportedFunctionThatsNotTheDefault is actually a valid exported function name before we start executing the script... So, probably a new Runner method would be required to get all of the exported function names or something like that.

[imiric]

This should technically be done (see my feat/1007-1300-env-tags-exec-options branch), but I'll hold off on creating a PR for it until after fix/1007-1283-context-cancelled (#1368) is merged, since it's based on it.

[na--]

Closed by #1428