[Keyserver] Open websocket connection with tunnelbroker

Summary:
Attempt to connect to tunnelbroker if available.
This is meant to be a "hello world" for establishing
a tunnelbroker connection.

https://linear.app/comm/issue/ENG-3766

Depends on D8476

Test Plan:

(cd services/tunnelbroker && cargo run &)
# should see message about db migration, then tunnelbroker message
(cd keyserver && yarn dev)

Reviewers: ashoat, jon, bartek, michal

Reviewed By: ashoat, michal

Subscribers: michal, bartek, tomek, atul

Differential Revision: https://phab.comm.dev/D7691