Auto merge of #4568 - Metaswitch:alt-registry-publish, r=withoutboats

Add support for publish to optionally take the index that can be used

This form part of alternative-registries RFC-2141, it allows crates to optionally specify which registries the crate can be be published to.

@carols10cents, one thing that I am unsure about is if there is a plan for publish to still provide index, or for registry to be provided instead. I thought that your general view was that we should move away from the index file. If we do need to map allowed registries to the index then there will be a small amount of extra work required once #4506 is merged.

@withoutboats, happy for this to be merged into your branch if you want, the main reason I did not base it on your branch was due to tests not working on there yet.