Factor out a GcStore interface

Starts progress on #5729.

The idea is that we should not have these default methods throwing
"unimplemented". This is a small step in that direction.

I kept `addTempRoot` because it is a no-op, rather than failure. Also,
as a practical matter, it is called all over the place, while doing
other tasks, so the downcasting would be annoying.

Maybe in the future I could move the "real" `addTempRoot` to `GcStore`,
and the existing usecases use a `tryAddTempRoot` wrapper to downcast or
do nothing, but I wasn't sure whether that was a good idea so with a
bias to less churn I didn't do it yet.

src/libstore/gc-store.cc

@@ -0,0 +1,13 @@
+#include "gc-store.hh"
+
+namespace nix {
+
+GcStore & requireGcStore(Store & store)
+{
+    auto * gcStore = dynamic_cast<GcStore *>(&store);
+    if (!gcStore)
+        throw UsageError("Garbage collection not supported by this store");
+    return *gcStore;
+}
+
+}