[Tunnelbroker] Add initial websocket connection support and connection map

Summary:
Add simple websocket implementation which can be used to easily onboard
keyserver support.

Part of:

• https://linear.app/comm/issue/ENG-3767
• https://linear.app/comm/issue/ENG-3766

Test Plan:

cd services/tunnelbroker
cargo run

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7610