Ensure the context-vars are propagated via executors

[jstriebel]

We use context-vars to manage the webknossos-url and tokens for the client. This should still work in executors.

From the https://github.com/scalableminds/webknossos-libs/pull/452#discussion_r729619271:

By default, it seems like contextvars are not propagated across executors (see this blog post). We might run need to adapt our executor abstractions to incorporate this.