KT-72838: [Gradle] Support websockets in webpack devServer proxy

[broundee]

Fixes KT-72838

Add ws parameter to webpack devServer proxy DSL in gradle plugin so that it is passed through to the generated webpack config.

The parameter is not listed in the webpack docs, but it's described in the underlying library's docs here:

option.ws: true/false: if you want to proxy websockets

[JSMonk]

@broundee Thank you so much for the contribution 🙏

[adam-enko]

It would be good to have a unit or functional test (just to verify the output is correct, it doesn't need to be a full integration test).

[broundee]

Added the test in 731ad6e.

[adam-enko]

Great, thanks for adding the test!

[adam-enko]

@JSMonk @bashor btw in future I would like to require all public-facing KGP JS code has KDoc. I'll let this one slide because we (@ilgonmic @JSMonk and me) already started adding KDoc to this class, so we'll add KDoc for ws in that MR.