firmare: start implementing session stat

Session-specific stat is a call required by the P2A bridge to allow the
host to configure its side of the equation.

Change-Id: I90570641cf9a6816fb91e8329582b882aef212cf
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/file_handler.hpp b/file_handler.hpp
index c419949..64be8bf 100644
--- a/file_handler.hpp
+++ b/file_handler.hpp
@@ -28,6 +28,7 @@
     void close() override;
     bool write(std::uint32_t offset,
                const std::vector<std::uint8_t>& data) override;
+    int getSize() override;
 
   private:
     /** the active hash path, ignore. */