Release all crates to crates.io on polkadot release

[athei]

As a first step towards having a nicer developer experience when using substrate/polkadot/cumulus we want to allow users to depend on it via crates.io. Git dependencies are just annoying and prevent everyone from releasing their own stuff to crates.io.

This is is just the first step and has a very limited scope. We don't care about narrowing down changes to minor and fix releases. We just bump the major versions of all crates. This is how it works:

• On master we still use git and path dependencies to connect substrate/polkadot/cumulus together
• We remove all versions from manifests on master for path and git dependencies
• On release we crate a tag on the release branches and release all the crates from there. The release process works like this:
  • Go through all the crates of all workspaces and set their version to the tag name. Don't deal with minor and fix version. Just use major versions: 38.0.0 -> 39.0.0.
  • This will re-add the versions to the dependencies in the manifest, too.
  • Then publish in this order: substrate->polkadot ->cumulus.
  • Within the workspace we need to start from the leaves and work our way up. cargo unleash might be able to do that.

This process would ideally be implemented as a github action. The workflow that uses it is then either manually triggered or on tag.