Add flutter support to nixpkgs

[mkg20001]

Motivation for this change

Implementation is dumpsterfire for now, but despite that it does work

I'll refine things until they are good enough. Reviews esp implementation tips appreciated

Testing can be done using "nix-build -A fluffychat"

Todos:

• cleanup mkFlutterApp
• remove hacks if possible
• possibly package more apps to check which deps we actually need
• make FOD stable

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[mkg20001]

Look and behold, it works ^^

Now's time to clean this frankensteins monster

[mkg20001]

I've started packaging some other apps in a branch https://github.com/NixOS/nixpkgs/compare/master...mkg20001:flutter-apps?expand=1 to see what needs to be adjusted for making this more general purpose

This should mostly be ready for merge now

[TilCreator]

I compiled Fluffychat Version 1.1.0.
it does work nicely. But encryption support is missing in 1.1.0 and 0.41.2, even though olm is in buildInputs, so not sure what the reason is there.

[colemickens]

There's AppFlowy which I have my eye on, but it's Flutter... and Rust: https://github.com/AppFlowy-IO/appflowy

[colemickens]

That slipped out a bit randomly, optimistically. My first thoughts were, wow, thank you for doing this. I can't wait.

[Misterio77]

Hey, i'm interested in building flutter web apps at my job using Nix. Could this work for web target too? If not, could you point at how you packaged the SDK stuff flutter downloads at runtime?

[mkg20001]

This is already big enough as it is and so that I don't slack another month on some improvements I'll just go ahead and merge this. The apps work, but haven't tested in-depth.