ContextExtension adds "stac_extensions" entry to LandingPage incorrectly

[lossyrob]

The extension mechanism causes a "stac_extensions" field to be put on the LandingPage, which incorrectly states the LandPage should have a "context" property:

{
    "type": "Catalog",
    "id": "stac-fastapi",
    "title": "stac-fastapi",
    "description": "stac-fastapi",
    "stac_version": "1.0.0",
    "conformsTo": [
        "https://api.stacspec.org/v1.0.0-beta.4/core",
        "https://api.stacspec.org/v1.0.0-beta.4/ogcapi-features",
        "https://api.stacspec.org/v1.0.0-beta.4/item-search/#context",
        "http://www.opengis.net/spec/ogcapi-features-1/1.0/conf/oas30",
        "http://www.opengis.net/spec/ogcapi-features-1/1.0/conf/geojson",
        "https://api.stacspec.org/v1.0.0-beta.4/item-search/#query",
        "https://api.stacspec.org/v1.0.0-beta.4/item-search/#sort",
        "http://www.opengis.net/spec/ogcapi-features-4/1.0/conf/simpletx",
        "http://www.opengis.net/spec/ogcapi-features-1/1.0/conf/core",
        "https://api.stacspec.org/v1.0.0-beta.4/ogcapi-features/extensions/transaction/",
        "https://api.stacspec.org/v1.0.0-beta.4/item-search/#fields",
        "https://api.stacspec.org/v1.0.0-beta.4/item-search"
    ],
    "links": [
        ...
    ],
    "stac_extensions": [
        "https://raw.githubusercontent.com/radiantearth/stac-api-spec/v1.0.0-beta.4/fragments/context/json-schema/schema.json"
    ]
}

The schema applies to responses to searches, not the LandingPage. This causes the Landing Page to fail validation as a Catalog.