Add cargoClean gradle task

[xianminx]

No description provided.

[ncalexan]

Thanks for the patch! I'm terribly busy, just coming off a few vacation days and catching up, but I'll try to process this within a week.

[ncalexan]

Some general comments.

1. Please include a note about this task in README.md.
2. Does this, in fact, work? I recall that the plugin sets the Cargo target directory based on the architecture. Does running cargo clean actually remove everything?
3. Does cargoClean${ARCHITECTURE} make sense? My preference is to generalize the cargo command invocation framework to support clean but also test and friends.
4. Finally, could you try to add a simple automated test that this in fact works? I'm aware that this adds to the work, but I'd like to improve the testing story as we add features.

I'm marking this as "Request changes" simply for the README.md request, which needs to be done before landing. But I'd like to get some discussion and hopefully some progress on the other questions and testing.

Thanks again!