fetchTree: add publicKeys as list input attribute

This adds the option of giving publicKeys as a list of attrsets
analogous to the flake fetcher input attribute.

src/libexpr/primops/fetchTree.cc

@@ -6,6 +6,7 @@
 #include "filetransfer.hh"
 #include "registry.hh"
 #include "url.hh"
+#include "flake/flake.hh"
 
 #include <ctime>
 #include <iomanip>
@@ -153,8 +154,13 @@ static void fetchTree(
             else if (attr.value->type() == nInt)
                 attrs.emplace(state.symbols[attr.name], uint64_t(attr.value->integer));
             else
-                state.debugThrowLastTrace(TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
-                    state.symbols[attr.name], showType(*attr.value)));
+                if (state.symbols[attr.name] == "publicKeys") {
+                    std::vector<nix::fetchers::PublicKey> publicKeys;
+                    parsePublicKeysList(state, attr, publicKeys);
+                    attrs.emplace(state.symbols[attr.name], publicKeys_to_string(publicKeys));
+                } else
+                    state.debugThrowLastTrace(TypeError("fetchTree argument '%s' is %s while a string, Boolean or integer is expected",
+                        state.symbols[attr.name], showType(*attr.value)));
         }
 
         if (!params.allowNameArgument)