Solves https://github.com/NixOS/nixpkgs/issues/146603
CoreDNS has support for plugins that are added at compile time. This
exposes an argument `externalPlugins` that will build coredns with
the specified plugins.
Example:
```
coredns-fanout = pkgs.coredns.override {
externalPlugins = [
{name = "fanout"; repo = "github.com/networkservicemesh/fanout"; version = "v1.9.1";}
];
vendorHash = "<SRI hash>";
};
```
95e66809de