Support Kerchunk indices embedded in STAC items (#33)

Closes: #32

This PR supports the following:

```python
import pystac
import xarray as xr

path = "https://raw.githubusercontent.com/stac-utils/xpystac/kerchunk/tests/data/data-cube-kerchunk-item-collection.json"
item_collection = pystac.ItemCollection.from_file(path)

ds = xr.open_dataset(item_collection)
ds
```

Author: jsignell

.gitignore

@@ -7,4 +7,4 @@ dist
 .pytest_cache
 .ruff_cache
 *.egg-info/
-.venv
+.venv

README.md

@@ -62,6 +62,21 @@ xr.open_dataset(asset, patch_url=planetary_computer.sign)
 ```
 ref: https://planetarycomputer.microsoft.com/docs/quickstarts/reading-zarr-data/
 
+
+Here is an example using the new approach of storing kerchunked metadata within the data-cube extension:
+
+```python
+import pystac
+import xarray as xr
+
+path = "https://raw.githubusercontent.com/stac-utils/xpystac/main/tests/data/data-cube-kerchunk-item-collection.json"
+item_collection = pystac.ItemCollection.from_file(path)
+
+ds = xr.open_dataset(item_collection)
+ds
+```
+
+
 ## Install
 
 ```bash

environment.yaml

@@ -0,0 +1,26 @@
+name: xpystac
+channels:
+  - conda-forge
+  - nodefaults
+dependencies:
+  - python
+  # required
+  - pystac
+  - xarray
+  # optional
+  - adlfs
+  - aiohttp
+  - fsspec
+  - kerchunk
+  - odc-stac
+  - planetary-computer
+  - pystac-client
+  - requests
+  - rioxarray
+  - stackstac
+  - urllib3<2 # temporary pin https://github.com/stac-utils/pystac-client/issues/509
+  - zarr
+  # testing
+  - pytest
+  - pytest-cov
+  - pytest-vcr

pyproject.toml

@@ -29,6 +29,7 @@ dev = [
     "adlfs>=2024.12.0",
     "aiohttp>=3.11.13",
     "fsspec>=2025.3.0",
+    "kerchunk>=0.2.7",
     "odc-stac>=0.3.11",
     "planetary-computer>=1.0.0",
     "pre-commit>=4.0.1",

tests/conftest.py

@@ -18,6 +18,12 @@ def simple_cog(simple_item) -> pystac.Asset:
     return asset
 
 
+@pytest.fixture(scope="module")
+def data_cube_kerchunk() -> pystac.ItemCollection:
+    path = "tests/data/data-cube-kerchunk-item-collection.json"
+    return pystac.ItemCollection.from_file(path)
+
+
 @pytest.fixture(scope="module")
 def simple_search() -> pystac_client.ItemSearch:
     client = pystac_client.Client.open(STAC_URLS["EARTH-SEARCH"])