`Filesystem` object store with a prefix leads to relative file paths in metadata files

[gruuya]

Consider the following change

--- a/datafusion_iceberg/examples/insert_table.rs
+++ b/datafusion_iceberg/examples/insert_table.rs
@@ -16,7 +16,7 @@ use std::sync::Arc;
 
 #[tokio::main]
 pub(crate) async fn main() {
-    let object_store = ObjectStoreBuilder::memory();
+    let object_store = ObjectStoreBuilder::filesystem("/Users/gruuya/warehouse");
 
     let catalog: Arc<dyn Catalog> = Arc::new(
         SqlCatalog::new("sqlite://", "test", object_store)

Whilst this works

warehouse % tree test
test
└── orders
    ├── data
    │   ├── day=18262
    │   │   └── ef5cd64c-7362-11f0-8000-b000b2e9ea86.parquet
    │   ├── day=18264
    │   │   └── ef60de2c-7362-11f0-b39d-e746482efe9c.parquet
    │   └── day=18294
    │       └── ef5cd656-7362-11f0-b39c-e0cc5b60165e.parquet
    └── metadata
        ├── 00000-b7b6b0c3-9c44-4e24-978d-34fb900e3246.metadata.json
        ├── 00001-3556f1e8-2425-4737-94a0-a86847927bdc.metadata.json
        ├── 00002-6686c10a-9ffd-4b64-98f7-3f670e9dd9e5.metadata.json
        ├── 27e64015-4164-4973-8700-95a822b858b2-m0.avro
        ├── 87eda61c-9f0a-4d94-8b7d-3086c4987914-m0.avro
        ├── snap-1477384046236171598-0-87eda61c-9f0a-4d94-8b7d-3086c4987914.avro
        ├── snap-4643981801262411697-0-27e64015-4164-4973-8700-95a822b858b2.avro
        └── version-hint.text

7 directories, 11 files

it breaks external readers, for instance in DuckDB

D select * from iceberg_scan('/Users/gruuya/warehouse/test/orders/metadata/00002-6686c10a-9ffd-4b64-98f7-3f670e9dd9e5.metadata.json');
IO Error:
Cannot open file "/test/orders/metadata/snap-1477384046236171598-0-87eda61c-9f0a-4d94-8b7d-3086c4987914.avro": No such file or directory

This is because the json and avro files end up having relative instead of absolute paths, which is not (yet) allowed by the spec

warehouse % jq . test/orders/metadata/00002-6686c10a-9ffd-4b64-98f7-3f670e9dd9e5.metadata.json | grep avro
      "manifest-list": "/test/orders/metadata/snap-1477384046236171598-0-87eda61c-9f0a-4d94-8b7d-3086c4987914.avro",
      "manifest-list": "/test/orders/metadata/snap-4643981801262411697-0-27e64015-4164-4973-8700-95a822b858b2.avro",
warehouse % fastavro test/orders/metadata/snap-1477384046236171598-0-87eda61c-9f0a-4d94-8b7d-3086c4987914.avro
{"manifest_path": "/test/orders/metadata/87eda61c-9f0a-4d94-8b7d-3086c4987914-m0.avro", "manifest_length": 7615, 
...
warehouse % fastavro test/orders/metadata/87eda61c-9f0a-4d94-8b7d-3086c4987914-m0.avro
... "data_file": {"content": 0, "file_path": "/test/orders/data/day=18262/ef5cd64c-7362-11f0-8000-b000b2e9ea86.parquet", "file_format": "PARQUET",
...

This is probably due to all other variants of ObjectStoreBuilder wrapping non-prefixed object stores, and accepting an absolute url in with_location of the table builder, whereas the Filesystem variant with a prefix works with relative paths.