fix: parse payload returned by k6

[e-fisher]

Fixes parsing issue when inspecting request payload in validator.

Steps to reproduce:

1. Create a generator from har file: test.k6.io login.har.zip
2. Validate script
3. Inspect POST request to login.php -> Payload
4. Payload is not displayed, parsing error in console.

When we parse a har file, we read x-www-form-urlencoded requests into JSON string, but k6 executable returns it as query string (key=value pairs). Added condition and function to parse query string.

[Llandy3d]

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