RFE: toolbox create --arch

**Is your feature request related to a problem? Please describe.**

I have a powerful workstation of CPU architecture A, and I am developing for deployment on CPU architecture B.

**Describe the solution you'd like**
I would like to use toolbox to manage a container for architecture B, so that I can use a "native" development toolchain (because machine A is faster, or because I don't otherwise have access to an architecture B machine).

**Describe alternatives you've considered**
- Virtual machines are too heavyweight.
- Cross-compilers merely produce code, they don't run it.

**Additional context**

This came from an internal slack conversation:

```
anyone know if it's possible to make toolbox create and manage a non-native-architecture container?
doesn't appear to be expected, but it can be worked around, since it accepts the image and container names explicitly, and that image can be a reference to an image of a non-default arch
for example the arm64 version of "fedora-toolbox:42", which including the registry part is "registry.fedoraproject.org/fedora-toolbox:42" skopeo inspect --raw docker://registry.fedoraproject.org/fedora-toolbox:42 lists an arm64 image with digest "sha256:1ae68d15077ea747834a20c834cf2617b058f3b452d417d37782d92287363dd9", and telling toolbox to use "registry.fedoraproject.org/fedora-toolbox@sha256:1ae68d15077ea747834a20c834cf2617b058f3b452d417d37782d92287363dd9", along with a manually-specified name for the container it bases on that image, worked over here
hah, nice! that didn't work on the first try because "copy" grabbed the http-prefixed url link rather than just the image path, but:
toolbox create -i registry.fedoraproject.org/fedora-toolbox@sha256:1ae68d15077ea747834a20c834cf2617b058f3b452d417d37782d92287363dd9 f42-arm
some enterprising soul should hook that up to toolbox create --arch or something
got an issue filed for that? i've used Flatpak runtimes for that in the past.
```