feat(tools/eaglemode): add plugin builder for eagle mode

Adds a buildPlugin function which can build Eagle Mode plugins that can
ultimately be linked into Eagle Mode using the existing etcDir function.

Change-Id: I338171779d3547faecbfb708fbaa78cd1cfd59ac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12387
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: emery <emery@dmz.rs>
Reviewed-by: azahi <azat@bahawi.net>

tools/eaglemode/plugins/example.nix

@@ -0,0 +1,17 @@
+{ depot, pkgs, ... }:
+
+let
+  em = depot.tools.eaglemode;
+  emSrc = with pkgs; srcOnly eaglemode;
+in
+em.buildPlugin {
+  name = "example";
+  version = "canon";
+
+  src = pkgs.runCommand "em-plugin-example-src" { } ''
+    set -ux
+    cp -r ${emSrc}/doc/examples/CppApiExamples/PluginExample $out
+  '';
+
+  target = "PlEx";
+}