feat: add Nova ecosystem metadata for Hex discovery (#21)

Adds extra metadata to the Hex package config so this plugin
is discoverable via `extra:nova,plugin` search queries.

Author: Taure

rebar.config

@@ -27,7 +27,10 @@
         {git, "https://github.com/Taure/rebar3_sbom.git", {branch, "feat/include-otp-components"}}}
 ]}.
 
-{hex, [{doc, #{provider => ex_doc}}]}.
+{hex, [
+    {doc, #{provider => ex_doc}},
+    {metadata, #{extra => #{<<"nova">> => <<"plugin">>}}}
+]}.
 
 {ex_doc, [
     {extras, [