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/dummy-store.cc

@@ -58,6 +58,15 @@ struct DummyStore : public virtual DummyStoreConfig, public virtual Store
         RepairFlag repair, CheckSigsFlag checkSigs) override
     { unsupported("addToStore"); }
 
+    virtual StorePath addToStoreFromDump(
+        Source & dump,
+        std::string_view name,
+        ContentAddressMethod method = FileIngestionMethod::Recursive,
+        HashAlgorithm hashAlgo = HashAlgorithm::SHA256,
+        const StorePathSet & references = StorePathSet(),
+        RepairFlag repair = NoRepair) override
+    { unsupported("addToStore"); }
+
     void narFromPath(const StorePath & path, Sink & sink) override
     { unsupported("narFromPath"); }
 

src/libstore/legacy-ssh-store.hh

@@ -69,6 +69,15 @@ struct LegacySSHStore : public virtual LegacySSHStoreConfig, public virtual Stor
         RepairFlag repair) override
     { unsupported("addToStore"); }
 
+    virtual StorePath addToStoreFromDump(
+        Source & dump,
+        std::string_view name,
+        ContentAddressMethod method = FileIngestionMethod::Recursive,
+        HashAlgorithm hashAlgo = HashAlgorithm::SHA256,
+        const StorePathSet & references = StorePathSet(),
+        RepairFlag repair = NoRepair) override
+    { unsupported("addToStore"); }
+
 private:
 
     void putBuildSettings(Connection & conn);

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