Add integration tests via shared repo

In addition, adding these integration tests uncovered a few issues, contained in single commits in this branch:

• Fix nullcontext code path in multi-endpoint streamer
• Fix type for dtype metadata in client

The repo containing the integration tests (https://git.ligo.org/ngdd/arrakis-integration-tests) were added using git subtree via:

git subtree add --prefix arrakis/tests/integration --squash -m 'add initial integration tests via arrakis-integration-tests' git@git.ligo.org:ngdd/arrakis-integration-tests.git main

After this had been set up, we can update the test repo by replacing the subtree add with a subtree pull with the same arguments.