authfs: Support write config/operation in fuse

Bug: 171279640
Test: atest
Test: tools/device-test.sh

Change-Id: Ic611f72d51a5522d9ec6e6fdc82c115b5782c4ac
diff --git a/authfs/src/fsverity/builder.rs b/authfs/src/fsverity/builder.rs
index 86c4969..94b9718 100644
--- a/authfs/src/fsverity/builder.rs
+++ b/authfs/src/fsverity/builder.rs
@@ -53,6 +53,11 @@
         Self { leaves: Vec::new(), file_size: 0 }
     }
 
+    /// Gets size of the file represented by `MerkleLeaves`.
+    pub fn file_size(&self) -> u64 {
+        self.file_size
+    }
+
     /// Updates the hash of the `index`-th leaf, and increase the size to `size_at_least` if the
     /// current size is smaller.
     pub fn update_hash(&mut self, index: usize, hash: &Sha256Hash, size_at_least: u64) {