Improve options exporting in HAR conversion and to JS runtime

[na--]

After merging #681, I realized that my advice to @MohanPrasathS there to use something like rt.Set("options", runner.GetOptions()) probably wasn't the best idea... The main reason for that was very similar to the reason I recently had to do this hack in for the options injection in the HAR file conversion. Basically, directly exporting the null-able types in lib.Options to JSON produces a lot of null and empty array/object values. Another slight drawback is that if a user adds their own non-k6 keys in the options object, they would have been overwritten and lost.

This PR adds a new method to lib.Options that allows us to flexibly traverse the lib.Options fields that are set. This improves both the HAR export format (it now uses unquoted JS keys) and the options propagation back to the JS runtime (no extra nulls and non-k6 user-defined options there won't be overwritten).

It's still not perfect... The current output of simply running script like this without any extra parameters:

export let options = {
	myOption: "test"
};

export default function (data) {
	console.log(JSON.stringify(options));
}

would be something like this:

{"myOption":"test","iterations":1,"tlsVersion":{"min":"","max":""},"systemTags":["proto","method","group","error","subproto","status","url","name","check","tls_version"]}

iterations and myOption are fine, tlsVersion appears due to this bug and systemTags is included due to a limitation in the way we (don't) track and handle complex default values...

Also, both this and in the original fix for #613 won't properly work when executing the script in the cloud (especially when there are multiple k6 instances), but fixing that would probably have to wait until we have proper cluster execution in k6 itself...

[codecov-io]

Codecov Report

Merging #713 into master will decrease coverage by 0.12%.
The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master     #713      +/-   ##
==========================================
- Coverage   64.38%   64.26%   -0.13%     
==========================================
  Files         101      101              
  Lines        8275     8302      +27     
==========================================
+ Hits         5328     5335       +7     
- Misses       2598     2617      +19     
- Partials      349      350       +1

Impacted Files	Coverage Δ
js/runner.go	78.81% <ø> (-0.11%)	⬇️
cmd/run.go	6.79% <0%> (ø)	⬆️
cmd/options.go	61.53% <0%> (-1.57%)	⬇️
lib/options.go	78.43% <0%> (-6.08%)	⬇️
converter/har/converter.go	17.55% <0%> (-0.35%)	⬇️
js/bundle.go	81.95% <85.71%> (-0.02%)	⬇️

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[robingustafsson]

LGTM