Declare DummyStoreConfig in a header

This will useful for unit tests.

src/libstore/dummy-store.cc

@@ -1,48 +1,15 @@
 #include "nix/store/store-registration.hh"
 #include "nix/util/callback.hh"
+#include "nix/store/dummy-store.hh"
 
 namespace nix {
 
-struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>, virtual StoreConfig
+std::string DummyStoreConfig::doc()
 {
-    using StoreConfig::StoreConfig;
-
-    DummyStoreConfig(std::string_view scheme, std::string_view authority, const Params & params)
-        : StoreConfig(params)
-    {
-        if (!authority.empty())
-            throw UsageError("`%s` store URIs must not contain an authority part %s", scheme, authority);
-    }
-
-    static const std::string name()
-    {
-        return "Dummy Store";
-    }
-
-    static std::string doc()
-    {
-        return
+    return
 #include "dummy-store.md"
-            ;
-    }
-
-    static StringSet uriSchemes()
-    {
-        return {"dummy"};
-    }
-
-    ref<Store> openStore() const override;
-
-    StoreReference getReference() const override
-    {
-        return {
-            .variant =
-                StoreReference::Specified{
-                    .scheme = *uriSchemes().begin(),
-                },
-        };
-    }
-};
+        ;
+}
 
 struct DummyStore : virtual Store
 {

src/libstore/include/nix/store/dummy-store.hh

@@ -0,0 +1,41 @@
+#include "nix/store/store-api.hh"
+
+namespace nix {
+
+struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>, virtual StoreConfig
+{
+    using StoreConfig::StoreConfig;
+
+    DummyStoreConfig(std::string_view scheme, std::string_view authority, const Params & params)
+        : StoreConfig(params)
+    {
+        if (!authority.empty())
+            throw UsageError("`%s` store URIs must not contain an authority part %s", scheme, authority);
+    }
+
+    static const std::string name()
+    {
+        return "Dummy Store";
+    }
+
+    static std::string doc();
+
+    static StringSet uriSchemes()
+    {
+        return {"dummy"};
+    }
+
+    ref<Store> openStore() const override;
+
+    StoreReference getReference() const override
+    {
+        return {
+            .variant =
+                StoreReference::Specified{
+                    .scheme = *uriSchemes().begin(),
+                },
+        };
+    }
+};
+
+} // namespace nix

src/libstore/include/nix/store/meson.build

@@ -34,6 +34,7 @@ headers = [ config_pub_h ] + files(
   'derived-path-map.hh',
   'derived-path.hh',
   'downstream-placeholder.hh',
+  'dummy-store.hh',
   'export-import.hh',
   'filetransfer.hh',
   'gc-store.hh',