Add support for local files to --log-output

[na--]

Feature Description

--log-output currently supports only none (disabled), stdout, stderr and loki.

Supporting local files should be simple and will be beneficial for debugging, or in CI environments, and it can also be used by log-forwarding tools to ship the logs pretty much anywhere. Combined with the logformat option, this feature should be pretty versatile.

Somewhat connected issue: #1939

[naoyukis]

Can I try this issue?

[na--]

Sure!

[codebien]

@na-- Do we expect to get some sort of rotation or is it something we would encourage to delegate to other tools?

[na--]

We should probably start without that, we can always add it later if there's demand for it.

[oleiade]

Some context is probably missing on my side, but what's the delta between redirecting stdout/stderr to a file from the command-line, and having k6 do it for us instead? As in, something along the lines of k6 run --log-output stderr samples/http_batch.js &> output.log.