Add a `StacCollection.search` method

*Purely as a convenience*, it'd be nice to have a `StacCollection.search` method that uses `pystac-client` to search an endpoint with a specific collection.

```python
cat = intake.open_stac_catalog("/path/to/catalog")
collection = cat["my-collection"]
collection.search(bbox=bbox)
```

The `.search` method would use pystac-client

1. Find the `link` with a `"rel": "search"`. Set that as the endpoint
2. Specify `collections=[self.id]`, to limit the search to just that collection.

---

I see now that intake's base Catalog class apparently defines a `search` method, which appears to do some kind of text-based search on the items. I suspect that most STAC users would expect search to behave like STAC search.