Updated plume for RS-68 versions with regeneratively cooled nozzles.

[Jay-NVB]

Always bothered me that sea-level hydrolox engines will too often use the RS-25 plume, when the large shock disc in said plume is a distinct product of the RS-25 being very overexpanded at sea level and sea-level-optimized nozzles will generally not produce such a shock disc (as evidenced by the RS-68 in particular, as well as the LE-9 engines at high power settings). So I removed the disc from the plume in a new Waterfall effect config, applied it to the RS-68K (and RS-800 too technically but I haven't used that variant), and was quite happy with the result.

This pull request is also me correcting some idiocy on my part; I had previously uploaded this config as a separate mod on Spacedock with "request addition to CKAN" checked, before joining the RO Discord server, where somebody informed me that I had not thought this through properly. Assuming the idea here isn't rejected, I'll take down the Spacedock page.

[Clayell]

It would be better to just have 1 RS68.cfg. Can you put your changes into there?

[Jay-NVB]

Moved the effect config into the part config. Not a huge fan of that solution though, kinda buries the effect config. I asked on the RO Discord where ROEngines stores its Waterfall effect configs, but it appears to only use the ones available in Waterfall itself.

I mention this because applying this effect config (rescaled, obviously) to M1-SL, HG-3-SL, or other such sea-level versions of hydrolox engines might be a thing. I'm not sure of that, because such engines might have setups like the LE-7 where it's still overexpanded for sea level but not as overexpanded as a vacuum engine, but the possibility is there. RS-68 and LE-9 are the only two I can think of off the top of my head that are definitely optimized for sea level, and ROEngines doesn't include an LE-9.