Add caching to cmip6 ingestion

This adds a global cache for holding cmip6's table_id to frequency and realm. It also adds a small utility to create some test cmip6 netcdf files with appropriate attribute data to be able to check this code path in our tests. The resulting files are committed so it doesn't need to be run frequently, only when test cases need to change.

freva/src/drs/ingest.rs

@@ -220,7 +220,7 @@ async fn ingest_dataset(
                 }
             };
 
-            let f = match dataset.metadata_from_path(&path) {
+            let f = match dataset.metadata_from_path(&path).await {
                 Ok(f) => f,
                 Err(e) => {
                     warn!("{} not a valid drs file, skipping:\n{:?}", path, e);