Implement "Transfer crate ownership" tool

[joao-paulo-parity]

In addition to reserving crates, we should also implement a GitHub Workflow for transferring crate ownership from Parity's developers to https://crates.io/users/parity-crate-owner. This will come in handy e.g. if a developer pre-reserved the crate for themselves initially and wants to move it to one of our repositories later, or if they forgot to reserve it through https://github.com/paritytech/releng-scripts#reserving-crates.

[chevdor]

cargo owner --add parity-crate-owner will work but only if the user is direct owner. Being part of the core-dev group does not allow ownership changes.

[joao-paulo-parity]

cargo owner --add parity-crate-owner will work but only if the user is direct owner.

That's precisely what this tool would be for. For example: I reserve the crate for a personal project initially, but later I want to bring that crate into Substrate. I left the implementation ideas out on purpose because I don't have a full picture of how it could be. Adding the owner should be fairly easy: accept the user's crates.io token as a workflow input and use it in cargo owner --token $INPUT_TOKEN --add parity-crate-owner. The murky part for me is how to accept the invite after, because apparently there isn't a cargo CLI command for that. It seems there's an API endpoint for accepting the invitation but I haven't figured out how to use it so far.

If I'm a Parity developer, there's only one step which is to run the workflow. If the owner is an external contributor, however, there's more indirection:

1 - Transfer the ownership to a Parity developer
2 - Parity developer accepts the invitation by email
3 - Parity developer runs the workflow to transfer the crate to parity-crate-owner

We can also support auto-accepting direct transfers to parity-crate-owner if they were sent from a Parity developer, but not otherwise. Again, I don't have a clear picture of how this could be implemented; perhaps by polling the invitations endpoint and running filters on that.