Allow integration with i18n providers

[curtisy1]

We should be able to fetch JSON translations from i18n providers as long as they support exporting. Ideally, this should be a plugin system where each provider is a standalone Nuget package that can be easily integrated.
The minimum supported features would be something along the lines of

• Fetch translations from API
• Do any custom transformations necessary (such as unzipping etc.)
• Add them to the build process as an additional source

For reference, here is a non-exhaustive list of providers

• Weblate
• Crowdin
• Localazy
• Locize
• POEditor
• Tolgee (@curtisy1)

They're all up for grabs, since maintaining all of these is too tedious. So when you need them, please contribute!

[curtisy1]

Since I integrated Tolgee as a PoC for #17, I'll tackle it and see how I can turn it into a plugin

[curtisy1]

Unfortunately, integrating this via separate NuGet packages is not as easy as I thought for the following reasons:
As described in the source generator docs, source generators run unordered this has a few caveats, especially the following:

• we can never be sure whether our plugins execute before our main generator
• we cannot access the files we downloaded from our i18n provider
• Init hooks are limited in a way that we cannot print helpful debug messages to the user

As such, the main repository now contains a plugins folder that has all existing plugins inside. When implementing, you can simply inherit from the abstract base class and override the Fetch method to return your desired files. For details, see #18 for an implementation