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/nix-collect-garbage/nix-collect-garbage.cc

@@ -1,4 +1,5 @@
 #include "store-api.hh"
+#include "gc-store.hh"
 #include "profiles.hh"
 #include "shared.hh"
 #include "globals.hh"
@@ -80,10 +81,11 @@ static int main_nix_collect_garbage(int argc, char * * argv)
         // Run the actual garbage collector.
         if (!dryRun) {
             auto store = openStore();
+            auto & gcStore = requireGcStore(*store);
             options.action = GCOptions::gcDeleteDead;
             GCResults results;
             PrintFreed freed(true, results);
-            store->collectGarbage(options, results);
+            gcStore.collectGarbage(options, results);
         }
 
         return 0;