Add collection paging

[gadomski]

Collection pagination was defined in radiantearth/stac-api-spec#221. It could be useful for large catalogs (e.g. the Planetary Computer).

[vincentsarago]

in v3.0, users are able to set the request model for the /collections endpoint (without needing the collection-search extension), so you can now enable pagination extension for the /collections endpoint.

Note: it's up to the user to set the correct conformance classes in the Client

api = app.StacApi(
    settings=ApiSettings(),
    client=...,
    collections_get_request_model=TokenPaginationExtension().GET,
    extensions=[
        TokenPaginationExtension()
    ],
)