bump the API version to 2; also remove some duplication in its definition

docs/source/configuration.rst

docs/source/interface.rst

src/_zkapauthorizer/_plugin.py

src/_zkapauthorizer/api.py

src/_zkapauthorizer/config.py

src/_zkapauthorizer/model.py

src/_zkapauthorizer/resource.py

src/_zkapauthorizer/storage_common.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_client_resource.py

src/_zkapauthorizer/tests/test_plugin.py

src/_zkapauthorizer/tests/test_storage_client.py

src/twisted/plugins/zkapauthorizer.py