Add collection search

Discussed in https://github.com/stac-utils/rustac-py/discussions/213

^{Originally posted by scottyhq January 5, 2026}
For APIs with a lot of STAC collections, you might not know your desired collection ID in advance. pystac_client has a get_collections() function to facilitate this kind of workflow.

I'm increasingly using rustac to search, but find myself using pystac_client only to quickly discover collection IDs and metadata. I'm wondering if such a function could be added to the API here? For example a replacement for this bit of code to iterate over pages of collections:

catalog = pystac_client.Client.open("https://cmr.earthdata.nasa.gov/stac/ASF")
collections = list(catalog.get_collections())
print(f"Found {len(collections)} collections")
# Found 153 collections
df_collections = pd.DataFrame([x.to_dict() for x in collections])
df_collections.head()

Start by reading the existing rustac-py client API and compare the requested behavior with pystac_client.Client.get_collections(), especially its paginated collection iteration. Define how the new function exposes collection metadata and when iteration is complete; the example using Client.open() and the expected count of 153 collections provide the acceptance target.