client.get_collections should check for other conformance classes

[philvarner]

in client.py, this code predicates behavior:

    def get_collection(self, collection_id: str) -> CollectionClient:
        if self._stac_io.conforms_to(ConformanceClasses.COLLECTIONS):

where

    COLLECTIONS = re.escape(
        "http://www.opengis.net/spec/ogcapi-features-1/1.0/conf/oas30"
    )

1. this is not the Collections conformance class -- it's the OAFeat OpenAPI 3.0 conformance class
2. It should actually be checking for either
   https://api.stacspec.org/v1.0.*/ogcapi-features or https://api.stacspec.org/v1.0.*/collections