Support reading runtime versions from asdf version manager config files

[asdf version manager](https://github.com/asdf-vm/asdf) is a popular version manager for multiple runtimes. This is a feature request to read its config files, [.tool-versions](https://asdf-vm.com/manage/configuration.html#tool-versions), for all runtimes.

For context see https://github.com/cloudflare/pages-build-image/discussions/1#discussion-4187652

Start by reading the linked asdf .tool-versions configuration documentation and the discussion referenced in the issue. Trace how the project currently discovers runtime versions, then determine the complete set of runtimes that must be read; done means versions are obtained from .tool-versions for all supported runtimes.

Written by the indexing model from the issue text.