Build and publish binaries after new release

[svix-mman]

Overview

This new workflow runs after a github release is published. It builds for 5 targets, and after all 5 builds are successful The newly built releases are published to the release tag

This serves 2 goals

1. In svix-webhooks CI, we use the openapi-codegen to generate our rust Lib. Now we don't need to re-compile (and take up cache space) every time we run the Libs/Rust CI
2. Dipping our toes into building/publishing binaries without cargo-dist

License

I added the MIT License so that the tar.xz/zip files will have a LICENSE file included, but this is not really needed.
If this is an issue, I can easily remove the License

How it works

After creating (and publishing) a new release from the GitHub UI, The workflow is triggered.

For the macos/linux builds I use cargo zigbuild, it uses the zig linker, allowing me to easily cross compile for arm/x86
I chose cargo zigbuild because this is what we (cargo-dist) already use in svix-webhooks, and it's very simple to use.

Homebrew is used to install needed dependencies

• On macos: zig cargo-zigbuild coreutils (sha256sum comes from coreutils)
• On linux: zig cargo-zigbuild

The resulting binary along with README.md and LICENSE is packaged in a openapi-codegen-{target triplet}.(tar.xz/zip) file.
On windows I package a zip file instead of a tar.xz (Some windows users may be unable to open a tar.xz archive

The sha256sum of the tar.xz/zip file is added to a file named openapi-codegen-{target triplet}.(tar.xz/zip).sha256

[svix-mman]

@tasn I added an MIT License in this PR
Let me know if that's ok

[svix-mman]

Maybe one day, but today is not the day