Support parallel CI jobs for crate publishing

[joao-paulo-parity]

PRs which touch lots of crates, for example paritytech/substrate#12768, take a long time to publish overall (example: https://gitlab.parity.io/parity/mirrors/substrate/-/jobs/2130765). We can improve the task's overall time to completion by splitting crates into groups and publishing those groups in parallel, perhaps using paritytech/substrate#12709 as reference for an implementation.

[joao-paulo-parity]

To publish a crate you need to publish all of its dependencies first, so it makes more sense as a serial processing operation which happens in the context of a single job rather than a distributed operation spread across parallel jobs. In the latter case they'd need to somehow synchronize between themselves so that Crate Y is only published from Job B after Crate X is published from Job A. Feasibility aside, I don't know if it's worthwhile to do that.

[joao-paulo-parity]

Changing the ticket to "question" because I'd like to first have the plan for a future implementation proposal before moving to the implementation itself