[dagit] Modify App initialization config
A couple things:
- The subscriptionParams config value is not currently used
- We want to have the flexibility to add headers to HttpLink
These headers will also be applied as the connectionParams. This is a little awkward, but seems like it might be the best way for us to get initialization info to the server.
Test Plan: Buildkite. Run Dagit, verify that all http and ws connections behave correctly.
Reviewers: dgibson, bengotow, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D9065