[Tech] Simplified adding runners (backend specific)

[unbelievableflavour]

This MR moves backend runner specific code into a single object so that it's a LOT easier to add new runners to the backend of the project. It's basically an addition to #4895,
But since that PR has already been reviewed I'd thought I'd offer it as a separate PR.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[CommandMC]

In general I'm very much in favor of this change. I've toyed with an idea like this in the past (having a "runner registry" that gets populated at runtime, to allow for a plugin-like runner interface), so this feels very familiar
Got a few small nitpicks

[CommandMC]

This is a little awkward. Out of scope here, but we should look into making this a branded type with a refine instead then