storage/Interface: add virtual method OpenFile()

This should replace most InputStream::Open() calls because it is a
chance to reuse existing resources (e.g. the NFS connection).  No such
optimization is implemented, currently (and the method is not yet used
by anybody).

src/storage/plugins/NfsStorage.cxx

@@ -11,6 +11,7 @@
 #include "lib/nfs/Lease.hxx"
 #include "lib/nfs/Connection.hxx"
 #include "lib/nfs/Glue.hxx"
+#include "input/InputStream.hxx"
 #include "fs/AllocatedPath.hxx"
 #include "thread/Mutex.hxx"
 #include "thread/Cond.hxx"
@@ -94,6 +95,8 @@ public:
 
 	[[nodiscard]] std::string_view MapToRelativeUTF8(std::string_view uri_utf8) const noexcept override;
 
+	InputStreamPtr OpenFile(std::string_view uri_utf8, Mutex &mutex) override;
+
 	/* virtual methods from NfsLease */
 	void OnNfsConnectionReady() noexcept final {
 		assert(state == State::CONNECTING);
@@ -246,6 +249,14 @@ NfsStorage::MapToRelativeUTF8(std::string_view uri_utf8) const noexcept
 	return PathTraitsUTF8::Relative(base, uri_utf8);
 }
 
+InputStreamPtr
+NfsStorage::OpenFile(std::string_view uri_utf8, Mutex &_mutex)
+{
+	// TODO create NfsInputStream directly
+	auto uri = MapUTF8(uri_utf8);
+	return InputStream::Open(uri.c_str(), _mutex);
+}
+
 static void
 Copy(StorageFileInfo &info, const struct nfs_stat_64 &st) noexcept
 {