Load completions from the host

**Is your feature request related to a problem? Please describe.**
Today it is not possible to load (ba)sh completions from the host, looks like **/usr/share/bash-completion** is banned.
```
⬢ [fedora-toolbox:39] > flatpak-spawn --host /usr/share/bash-completion/completions/podman
Portal call failed: Failed to start command: Failed to execute child process “/usr/share/bash-completion/completions/podman” (Permission denied)
```

**Describe the solution you'd like**
I would like to be able to import those completions without the need of installing the corresponding packets on each toolbox container.

Start by reproducing the flatpak-spawn --host command against /usr/share/bash-completion/completions/podman and trace how toolbox handles host command access. Identify the existing host/container boundary involved in completion loading. Done means bash completions can be imported from the host without installing the corresponding packages in each toolbox container.

Written by the indexing model from the issue text.