feat(lib): Reintroduce engine version file

Right now if you boot up MeiliSearch and point it to a DB directory created with a previous version of MeiliSearch the existing indexes will be deleted. This used to be prevented by a startup check which would compare the current engine version vs what was stored in the DB directory's version file, but this functionality seems to have been lost after a few refactorings of the code.
In order to go back to the old behavior we'll need to reintroduce the VERSION file that used to be present; I considered reusing the metadata.json file used in the dumps feature, but this seemed like the simpler and more approach. As the intent is just to restore functionality, the implementation is quite basic. I imagine that in the future we could build on this and do things like compatibility across major/minor versions and even migrating between formats.

This PR was made thanks to @mbStavola

Closes #1840

meilisearch-lib/src/snapshot.rs

@@ -9,6 +9,7 @@ use tokio::time::sleep;
 use walkdir::WalkDir;
 
 use crate::compression::from_tar_gz;
+use crate::index_controller::versioning::VERSION_FILE_NAME;
 use crate::tasks::task::Job;
 use crate::tasks::TaskStore;
 
@@ -102,6 +103,7 @@ impl SnapshotJob {
         let temp_snapshot_dir = tempfile::tempdir()?;
         let temp_snapshot_path = temp_snapshot_dir.path();
 
+        self.snapshot_version_file(temp_snapshot_path)?;
         self.snapshot_meta_env(temp_snapshot_path)?;
         self.snapshot_file_store(temp_snapshot_path)?;
         self.snapshot_indexes(temp_snapshot_path)?;
@@ -133,6 +135,15 @@ impl SnapshotJob {
         Ok(())
     }
 
+    fn snapshot_version_file(&self, path: &Path) -> anyhow::Result<()> {
+        let dst = path.join(VERSION_FILE_NAME);
+        let src = self.src_path.join(VERSION_FILE_NAME);
+
+        fs::copy(src, dst)?;
+
+        Ok(())
+    }
+
     fn snapshot_meta_env(&self, path: &Path) -> anyhow::Result<()> {
         let mut options = heed::EnvOpenOptions::new();
         options.map_size(self.meta_env_size);