User configurable plugins manifest URL for Plugin Store #2178

Provide the option for an user-configurable manifest URL (like an option between github, jsdelivr or user-defined) which will have the following additional benefits:

1. it will be possible to have "unofficial" plugin communities, which means the whole ecosystem is a bit more open
2. it will make it possible to do full integration testing of plugins: for example, before opening a PR to the manifest repository to add a new plugin, I can point my flow-launcher to my fork repository and test that a complete install/update/etc works as expected.