User configurable plugins manifest URL for Plugin Store

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.

The issue names no files, tests, or entry points. Start by locating the Plugin Store manifest URL configuration and the plugin install/update flow, then determine how GitHub, jsdelivr, and user-defined sources should be selected. Done means a user can configure a manifest URL and complete plugin install, update, and related flows from that source.

Written by the indexing model from the issue text.