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/local-fs-store.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "store-api.hh"
+#include "gc-store.hh"
 
 namespace nix {
 
@@ -23,7 +24,7 @@ struct LocalFSStoreConfig : virtual StoreConfig
         "physical path to the Nix store"};
 };
 
-class LocalFSStore : public virtual LocalFSStoreConfig, public virtual Store
+class LocalFSStore : public virtual LocalFSStoreConfig, public virtual Store, virtual GcStore
 {
 public: