TextHashMethod -> TextIngestionMethod, gate with XP feature

I suppose we can use `dynamic-derivations` for the few things we neeed.

src/libutil/experimental-features.cc

@@ -12,7 +12,7 @@ struct ExperimentalFeatureDetails
     std::string_view description;
 };
 
-constexpr std::array<ExperimentalFeatureDetails, 12> xpFeatureDetails = {{
+constexpr std::array<ExperimentalFeatureDetails, 13> xpFeatureDetails = {{
     {
         .tag = Xp::CaDerivations,
         .name = "ca-derivations",
@@ -199,6 +199,16 @@ constexpr std::array<ExperimentalFeatureDetails, 12> xpFeatureDetails = {{
             networking.
         )",
     },
+    {
+        .tag = Xp::DynamicDerivations,
+        .name = "dynamic-derivations",
+        .description = R"(
+            Allow the use of a few things related to dynamic derivations:
+
+              - "text hashing" derivation outputs, so we can build .drv
+                files.
+        )",
+    },
 }};
 
 static_assert(

src/libutil/experimental-features.hh

@@ -29,6 +29,7 @@ enum struct ExperimentalFeature
     Cgroups,
     DiscardReferences,
     DaemonTrustOverride,
+    DynamicDerivations,
 };
 
 /**