Set fallback region (#797)

* Set fallback region

Closes #793

I confirmed that this works
1. by deleting my default region in `.aws/config`.
2. Creating a new repo on tigris. This action fails without this change.

* lint fix

Author: dcherian

design-docs/009-tigris-support.md

@@ -37,7 +37,7 @@ By default Icechunk should be perfectly safe and consistent, even at the price o
   * All writes and reads use the `X-Tigris-Regions` header set to the initialized region.
   * All reads use the `Cache-Control:no-cache` header.
   * This will make Icechunk safe, but it will direct all writes and reads to a single region, which may cause performance degradation.
-  
+
 * Setting a `Storage` configuration variable to "`unsafe_read_from_local_region: True`":
   * Region is ignored if passed
   * No `X-Tigris-Regions` header is passed.

icechunk/src/storage/s3.rs

@@ -78,6 +78,16 @@ pub async fn mk_client(config: &S3Options, credentials: S3Credentials) -> Client
         .unwrap_or_else(RegionProviderChain::default_provider);
 
     let endpoint = config.endpoint_url.clone();
+    let region = if endpoint.is_some() {
+        // GH793, the S3 SDK requires a region even though it may not make sense
+        // for S3-compatible object stores like Tigris or Ceph.
+        // So we set a fake region, using the `endpoint_url` as a sign that
+        // we are not talking to real S3
+        region.or_else(Region::new("region-was-not-set"))
+    } else {
+        region
+    };
+
     #[allow(clippy::unwrap_used)]
     let app_name = AppName::new("icechunk").unwrap();
     let mut aws_config = aws_config::defaults(BehaviorVersion::v2024_03_28())