Adding --skip-existing option to publish

[danielvschoor]

I had a random network failure on one of my Github Actions publish jobs, so I had to re-run the workflow, but all the other jobs failed due to the fact that the wheels already existed on Pypi.

Would it be feasible to add a --skip-existing arg to maturin, that works exactly the same as in Twine? I don't mind doing a PR for it