[apkverify] Refactor signer and apk section extraction for reuse

Bug: 247689066
Test: libapkverify.integration_test
Change-Id: I6ae2d89a72d588a05fe5c19bc2afd4d7c8c2e39b
diff --git a/libs/apkverify/src/bytes_ext.rs b/libs/apkverify/src/bytes_ext.rs
index 8fb36ee..8a7badf 100644
--- a/libs/apkverify/src/bytes_ext.rs
+++ b/libs/apkverify/src/bytes_ext.rs
@@ -32,6 +32,13 @@
     }
 }
 
+impl<T> LengthPrefixed<T> {
+    /// Consumes the `LengthPrefixed` instance, returning the wrapped value.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
+
 pub trait BytesExt {
     fn read<T: ReadFromBytes>(&mut self) -> Result<T>;
 }