Add asdf support

[sambhav]

What would you like to be added:

syft should add support for asdf. asdf is a generic package manager that can be used to install various tools. See https://github.com/asdf-vm/asdf

Why is this needed:

Using this syft may be able to capture versions of tools that are not currently captured by syft. Things like the version of python, ruby, node etc.

Additional context:

[sambhav]

See https://github.com/asdf-vm/asdf-plugins for the full list of binaries supported by asdf.

[kzantow]

This seems like a great idea -- we welcome PRs!

[wagoodman]

For anyone picking this up, it sounds like this would be a good fit for cataloging directories which look for declarations of packages and not container images which look for installations of packages.