STYLE: Move getKVstoreDriver(std::string) into unnamed namespace

Allowed `getKVstoreDriver` to be called by other functions from the unnamed
namespace more easily.

Following C++ Core Guidelines, May 11, 2024, "Use an unnamed (anonymous)
namespace for all internal/non-exported entities",
http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf22-use-an-unnamed-anonymous-namespace-for-all-internalnon-exported-entities

src/itkOMEZarrNGFFImageIO.cxx

@@ -166,6 +166,26 @@ itkToTensorstoreComponentType(const IOComponentEnum itkComponentType)
   }
 }
 
+// Returns TensorStore KvStore driver name appropriate for this path.
+// Options are file, zip. TODO: http, gcs (GoogleCouldStorage), etc.
+std::string
+getKVstoreDriver(std::string path)
+{
+  if (path.size() < 4)
+  {
+    return "file";
+  }
+  if (path.substr(0, 4) == "http")
+  { // http or https
+    return "http";
+  }
+  if (path.substr(path.size() - 4) == ".zip" || path.substr(path.size() - 7) == ".memory")
+  {
+    return "zip_memory";
+  }
+  return "file";
+}
+
 template <typename TPixel>
 void
 ReadFromStore(const tensorstore::TensorStore<> & store, const ImageIORegion & storeIORegion, TPixel * buffer)
@@ -293,26 +313,6 @@ OMEZarrNGFFImageIO::PrintSelf(std::ostream & os, Indent indent) const
   os << indent << "ChannelIndex: " << m_ChannelIndex << std::endl;
 }
 
-// Returns TensorStore KvStore driver name appropriate for this path.
-// Options are file, zip. TODO: http, gcs (GoogleCouldStorage), etc.
-std::string
-getKVstoreDriver(std::string path)
-{
-  if (path.size() < 4)
-  {
-    return "file";
-  }
-  if (path.substr(0, 4) == "http")
-  { // http or https
-    return "http";
-  }
-  if (path.substr(path.size() - 4) == ".zip" || path.substr(path.size() - 7) == ".memory")
-  {
-    return "zip_memory";
-  }
-  return "file";
-}
-
 // JSON file path, e.g. "C:/Dev/ITKIOOMEZarrNGFF/v0.4/cyx.ome.zarr/.zgroup"
 void
 writeJson(nlohmann::json json, std::string path, std::string driver)