Allow users to download and install arbitrary snap versions, like developers can.

[`{third: "Beedell", first: "Roke"}`{.JSON5}]

As https://forum.snapcraft.io/t/how-to-install-specific-version-of-snap/39254/3?u=beedellrokejulianloc explains, although the current answers at https://askubuntu.com/q/1268615/1002900 explain how to install versions of packages which provide channels for specific versions, this isn't applicable for most packages. Additionally, none actually answer how to install a specific *version* of a snap package on a channel, instead of how to merely track a different (in their examples, versioned) channel.

I request the ability to install arbitrary versions because for diagnostic purposes — usually creating bug reports, but also when specific package version is programmatically useful — I need to be able to choose a specific version irrespective of the channels available.

As an example,

```.PS1
snap install powershell --channel=latest/stable
```

shall currently install `7.4.1`, but when that changes, the command no longer installs `7.4.1`. This package does not provide versioned channels. How do I force it to forever install `7.4.1`? This specific example is a demonstration of the necessity of this feature, per https://bugs.kde.org/show_bug.cgi?id=482881#c0.

For comparison, I can install a specific version of a Flatpak package using

```.PS1
flatpak install https://dl.flathub.org/repo/appstream/com.github.Murmele.Gittyup.flatpakref && `
flatpak update app/com.github.Murmele.Gittyup/x86_64/stable --commit=7e1c9370a1a462478b7fac34aac11882cb62768886aa23fe4fd36fb23c17e19b
```

which makes the lack of this feature here all the more perplexing.

[`{third: "Beedell", first: "Roke"}`{.JSON5} (rokejulianlockhart)]

Set to “Fix released” per https://forum.snapcraft.io/t/how-to-install-specific-version-of-snap/39254/6?u=beedellrokejulianloc.