Support .tool-versions for asdf tool management
As a user of `asdf` for development, it helps that if I switch to the `cli` project directory, that a golang version will be automatically picked for me.
I can do this by creating a `.tool-versions` file in the project directory (it'll be git ignored) but I feel it helps other contributors as well if they use `asdf` if we have a `.tool-versions` file in the repository.
issue