First step towards required server-side ZKAP validation

docs/source/configuration.rst

setup.py

src/_zkapauthorizer/_plugin.py

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/storage_common.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_plugin.py

src/_zkapauthorizer/tests/test_storage_protocol.py

src/_zkapauthorizer/tests/test_storage_server.py

src/_zkapauthorizer/tests/testing-signing.key