Include fetcher input scheme info in the CLI dump

Leverages the previous commit.

src/libfetchers/fetchers.cc

@@ -19,6 +19,19 @@ void registerInputScheme(std::shared_ptr<InputScheme> && inputScheme)
     inputSchemes->insert_or_assign(schemeName, std::move(inputScheme));
 }
 
+nlohmann::json dumpRegisterInputSchemeInfo() {
+    using nlohmann::json;
+
+    auto res = json::object();
+
+    for (auto & [name, scheme] : *inputSchemes) {
+        auto & r = res[name] = json::object();
+        r["allowedAttrs"] = scheme->allowedAttrs();
+    }
+
+    return res;
+}
+
 Input Input::fromURL(const std::string & url, bool requireTree)
 {
     return fromURL(parseURL(url), requireTree);

src/libfetchers/fetchers.hh

@@ -8,6 +8,7 @@
 #include "url.hh"
 
 #include <memory>
+#include <nlohmann/json_fwd.hpp>
 
 namespace nix { class Store; }
 
@@ -170,4 +171,6 @@ struct InputScheme
 
 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);
 
+nlohmann::json dumpRegisterInputSchemeInfo();
+
 }

src/nix/main.cc

@@ -188,6 +188,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs, virtual RootArgs
             j["experimentalFeature"] = storeConfig->experimentalFeature();
         }
         res["stores"] = std::move(stores);
+        res["fetchers"] = fetchers::dumpRegisterInputSchemeInfo();
 
         return res.dump();
     }