chore: dry run release workflow for the gen2-migration branch

[iliapolo]

Description of changes

Added a GitHub workflow that builds the @aws-amplify/cli-internal package in the gen2-migration branch and publishes it to NPM.

• Publish under a new package name so we don't collide with the regular package release: @aws-amplify/cli-internal-gen2-migration-alpha
• Give the package an -alpha suffix and a 0.x version since it is experimental.

Currently the workflow only performs a dry-run; once we see the expected result we will remove the dry-run option.

The publishing itself will leverage NPM trusted publishing after an initial manual release with a token.

Why not release the @aws-amplify/cli package?

The @aws-amplify/cli package is a wrapper on top of @aws-amplify/cli-internal that dynamically installs an amplify binary based on the operating system of the user.

Those binaries are built and published via the standard CLI release process, which we currently would like to avoid. The consequence of this is creating a standard NPM package with many dependencies - instead of a bundled executable binary. For the purposes of this experimental release - this is ok.

Note that contrary to what the name suggests, the @aws-amplify/cli-internal is actually public and already published to NPM, so we are not publishing anything we don't already publish.

Description of how you validated changes

Manually executed the steps locally on a clean clone of the repo.

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.