Introduce a file access interface for reading additional files.

src/context/file_access_interface.rs

@@ -0,0 +1,13 @@
+use std::path::Path;
+
+pub trait FileAccessInterface {
+    fn read_file<P: AsRef<Path>>(&self, path: P) -> Result<String, Box<dyn std::error::Error>>;
+}
+
+pub struct LocalFileAccessInterface;
+
+impl FileAccessInterface for LocalFileAccessInterface {
+    fn read_file<P: AsRef<Path>>(&self, path: P) -> Result<String, Box<dyn std::error::Error>> {
+        Ok(std::fs::read_to_string(path)?)
+    }
+}