Search for other executable on PATH if no .tool-versions and no global

### Is your feature request related to a problem? Please describe

In my scenario I have the tool `terraform` installed globally via Homebrew where I always get the latest version and completion automatically. Sometimes I need an older version of Terraform in that case I want to use asdf. I would like Asdf to look for another executable on PATH if no .tool-versions and no global version is available before erroring out.

### Describe the proposed solution

Adjust asdf maybe via a flag to look for another fitting executable on PATH if no .tool-versions can be found and no global version has been set for a package.

```shell
if tool_version_file_somewhere; then
# continue as usual
elif global_version_set; then
# continue as usual
elif exe_in_path_other_than_asdf_shim; then
# call this exe with args
else
# print error "No version is set for command..."
fi
```

### Describe similar `asdf` features and why they are not sufficient

Using the globals feature of Asdf is not sufficient in comparison to Homebrew. Homebrew handles automatic updates and completion.

### Describe other workarounds you've considered

- Using asdf globals instead of Homebrew for packages I want to always keep up to date and globally available.
- Monkeypatch asdf. Could be possible if overall modification is feasible because I use Ansible to bootstrap my environments.

Start by tracing how the asdf shim handles a missing .tool-versions file and absent global version, including where it emits "No version is set for command." Compare that flow with the proposed PATH fallback and flag behavior; done means an executable other than the asdf shim is selected only in the requested case without changing normal version resolution.

Written by the indexing model from the issue text.