feat!: add support for asdf >= 0.16

[Dabolus]

This PR adds support for asdf >= 0.16, which has been rewritten from scratch in Go and is no longer simply cloneable from git. It does so by downloading the pre-built release from GitHub for the correct platform and architecture.

The change adds support for a new optional parameter, asdf_version, which can be used to set a specific version to download and install. If not provided, the latest release will be downloaded by default.

I decided to add a new parameter because the existing one (asdf_branch) allows to specify not only a given version tag, but also any kind of branch. To support the same parameter with the rewritten asdf, the action should build asdf from source, but I don't think that's needed.

As of today, asdf_branch can be used to download the older, bash-based versions of asdf, so it's still possible to use the actions with those versions by simply adding e.g. asdf_branch: v0.15.0.

Note that, even though the older asdf versions are still downloadable using the parameter above, by default the new behavior is used, so this will most probably need a major version bump.

Let me know if the implementation makes sense and/or if I missed something.

Fixes #587