Add goreleaser script for releasing binaries to our Alpha customers

[fatih]

This PR adds a new script that publishes alpha releases for our customers. Once the script is merged, it'll allow us to publish updated binaries to the alpha repository. The goal is to add the alpha customers as collaborators and download the binary from the release tab. The alpha repository will also be a one-stop place for them to file issues and give us feedback.

The goreleaser configuration file is deliberately not tied to the CI system as we want to manually publish releases with a proper changelog until the CLI repository is public. The alpha releases are published to the repository https://github.com/planetscale/psdb-alpha. (nit: I've pinged @lkburns whether we can use https://github.com/planetscale/alpha, I'll update this issue if the repository URL changes)

The command to publish the release is:

GORELEASER_CURRENT_TAG=v0.2.0 goreleaser release --config .goreleaser-alpha.yml  --rm-dist --skip-validate --release-notes changelog.md

There are a couple of things that need some mentioning:

• To run the command, you need to specify a GITHUB_TOKEN environment variable with access to the alpha repository.
• Tag: If we don't specify a tag (via GORELEASER_CURRENT_TAG), the goreleaser will use the tag defined from this repo (github.com/planetscale/cli/). Let us use the tag from the CLI repo as it will help to debug issues. The reason is, we're going to ask for them for a version, and they will provide one with pscale --version. It will be helpful to observe that the version matches the version from our repo.
• We skip the changelog because we don't want to leak internal information from the commit messages. Second, we also want to provide a more human-friendly, useful changelog. Hence we're going to craft the changelogs ourselves via the --release-notes flag.
• We're skipping the README.md and License files. The Readme file includes internal information from the CLI repository, so we shouldn't include it accidentally. And the License file isn't needed for the Alpha customer as it's not open-source yet.

/xref https://github.com/planetscale/project-big-bang/issues/157