Make it possible to set source subdataset

When driver data is a dict with `subdataset` key,
set that on RasterSource.

odc/loader/testing/fixtures.py

@@ -96,9 +96,11 @@ def __init__(
         self,
         group_md: RasterGroupMetadata,
         driver_data,
+        add_subdataset: bool = False,
     ):
         self._group_md = group_md
         self._driver_data = driver_data
+        self._add_subdataset = add_subdataset
 
     def extract(self, md: Any) -> RasterGroupMetadata:
         assert md is not None
@@ -107,12 +109,19 @@ def extract(self, md: Any) -> RasterGroupMetadata:
     def driver_data(self, md, band_key: BandKey) -> Any:
         assert md is not None
         name, _ = band_key
+
+        def _patch(x):
+            if not isinstance(x, dict) or self._add_subdataset is False:
+                return x
+            return {"subdataset": name, **x}
+
         if isinstance(self._driver_data, dict):
             if name in self._driver_data:
-                return self._driver_data[name]
+                return _patch(self._driver_data[name])
             if band_key in self._driver_data:
-                return self._driver_data[band_key]
-        return self._driver_data
+                return _patch(self._driver_data[band_key])
+
+        return _patch(self._driver_data)
 
 
 class FakeReader:

odc/stac/_mdtools.py

@@ -692,8 +692,11 @@ def _get_grid(grid_name: str, asset: pystac.asset.Asset) -> GeoBox:
             )  # pragma: no cover (https://github.com/stac-utils/pystac/issues/754)
 
         driver_data: Any = None
+        subdataset: str | None = None
         if md_plugin is not None:
             driver_data = md_plugin.driver_data(asset, bk)
+            if isinstance(driver_data, dict):
+                subdataset = driver_data.get("subdataset", None)
 
         # Assumption: if extra dims are defined then asset bands are loaded into 3d+ array
         if meta.extra_dims:
@@ -702,6 +705,7 @@ def _get_grid(grid_name: str, asset: pystac.asset.Asset) -> GeoBox:
         bands[bk] = RasterSource(
             uri=uri,
             band=band_idx,
+            subdataset=subdataset,
             geobox=geobox,
             meta=meta,
             driver_data=driver_data,

tests/test_mdtools.py

@@ -275,6 +275,16 @@ def test_parse_item_with_plugin():
     assert pit.collection["b2"].data_type == "float32"
     assert pit["b1"].driver_data == {"foo": "bar"}
     assert pit["b2"].driver_data == {"foo": "bar"}
+    assert pit["b1"].subdataset is None
+    assert pit["b2"].subdataset is None
+
+    md_plugin = FakeMDPlugin(group_md, {"foo": "bar"}, add_subdataset=True)
+    pit = parse_item(item, md_plugin=md_plugin)
+    assert isinstance(pit, ParsedItem)
+    assert pit.collection["b1"].data_type == "uint8"
+    assert pit["b1"].driver_data == {"foo": "bar", "subdataset": "AA"}
+    assert pit["b1"].subdataset == "AA"
+    assert pit["b2"].subdataset == "AA"
 
 
 def test_noassets_case(no_bands_stac):