feat[UnifiedPush]: notifications for multiple accounts

[baltevl]

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

• Code formatting and import sorting has been done with dart format lib/ test/ and dart run import_sorter:main --no-comments
• The commit message uses the format of Conventional Commits
• The commit message describes what has been changed, why it has been changed and how it has been changed
• Every new feature or change of the design/GUI is linked to an approved design proposal in an issue
• Every new feature in the app or the build system has a strategy how this will be tested and maintained from now on for every release, e.g. a volunteer who takes over maintainership

Pull Request has been tested on:

• Android (emulator only for now)
• iOS
• Browser (Chromium based)
• Browser (Firefox based)
• Browser (WebKit based)
• Desktop Linux
• Desktop Windows
• Desktop macOS

Hey, again this is another attempt to bring support for notifications for multiple accounts to fluffy. As with the previous one this is only a draft for now and I'd appreciate any feedback :)

[baltevl]

ping @krille-chan. First of all i don't want to bother you with this. I really what this to happen but I totally understand if you don't have the time, motivation and/or resources to review this and give feedback :)
Now to the technical part: I rewrote my pr with significantly fewer structural changes. It is at a stage, i'd like to hear feedback and hear your opinion about how it fits in the project