clanModules/mycelium: init mycelium

This adds a clanModule for `mycelium`.
`Mycelium` is an IPv6 overlay network written in Rust.
Each node that joins the overlay network will receive an overlay network IP in the 400::/7 range.

An example configuration might look like this in the inventory:

```nix
mycelium.default = {
  roles.peer.machines = [
    "berlin"
    "munich"
  ];
  config = {
    topLevelDomain = "m";
    openFirewall = true;
    addHostedPublicNodes = true;
  };
};
```

This will add the machines named `berlin` and `munich` to the `mycelium` vpn.
And will also set the toplevel domain of the mycelium vpn to `m`, meaning the
machines are now reachable via `berlin.m` and `munich.m`.

clanModules/flake-module.nix

@@ -26,6 +26,7 @@ in
     matrix-synapse = ./matrix-synapse;
     moonlight = ./moonlight;
     mumble = ./mumble;
+    mycelium = ./mycelium;
     nginx = ./nginx;
     packages = ./packages;
     postgresql = ./postgresql;