feat: accept blob URI writes (#3954)

#3528 added blob declarations and binary coercion. String values were
still rejected. They now coerce to the blob `uri` child.

```python
table.add([{"id": 1, "image": "s3://bucket/media/cat.jpg"}])

payload = table.fetch_blobs("image", table.search().to_arrow())
```

A URI under a registered base writes with no extra options. An
unregistered URI fails. `allow_external_blob_outside_bases` is a local
escape hatch that stores an absolute URI. It does not register a base.
Remote `add` rejects that flag before making a request. String input
still coerces and is sent as a `uri` struct.

`add_bases` is a follow-up. `merge_insert` does not coerce string blob
input.

### Testing

- `cargo test -p lancedb --test blob_integration`
- `cargo test -p lancedb blob_coerce`
- `cargo test -p lancedb --features remote --lib
add_rejects_external_blob_flag add_string_blob_becomes_uri_struct`
- `cd python && uv run --extra tests pytest python/tests/test_blob.py -k
uri -q`

Co-authored-by: Xuanwo <github@xuanwo.io>

python/src/table.rs

@@ -780,15 +780,19 @@ impl Table {
         })
     }
 
-    #[pyo3(signature = (data, mode, progress=None, write_parallelism=None))]
+    #[pyo3(signature = (data, mode, progress=None, write_parallelism=None, allow_external_blob_outside_bases=false))]
     pub fn add<'a>(
         self_: PyRef<'a, Self>,
         data: PyScannable,
         mode: String,
         progress: Option<Py<PyAny>>,
         write_parallelism: Option<usize>,
+        allow_external_blob_outside_bases: bool,
     ) -> PyResult<Bound<'a, PyAny>> {
-        let mut op = self_.inner_ref()?.add(data);
+        let mut op = self_
+            .inner_ref()?
+            .add(data)
+            .allow_external_blob_outside_bases(allow_external_blob_outside_bases);
         if mode == "append" {
             op = op.mode(AddDataMode::Append);
         } else if mode == "overwrite" {