Push addToStoreFromDump unsupported(...) down Store class hierarchy

Instead of having it be the default method in `Store` itself, have it be
the implementation in `DummyStore` and `LegacySSHStore`. Then just the
implementations which fail to provide the method pay the "penalty" of
dealing with the icky `unimplemented` function for non-compliance.

Picks up where #8217. Getting close to no `unsupported` in the `Store`
interface itself!

More progress on issue #5729.

src/libstore/store-api.hh

@@ -466,8 +466,7 @@ public:
         ContentAddressMethod method = FileIngestionMethod::Recursive,
         HashAlgorithm hashAlgo = HashAlgorithm::SHA256,
         const StorePathSet & references = StorePathSet(),
-        RepairFlag repair = NoRepair)
-    { unsupported("addToStoreFromDump"); }
+        RepairFlag repair = NoRepair) = 0;
 
     /**
      * Add a mapping indicating that `deriver!outputName` maps to the output path