[lib] introduce Tunnelbroker Context

Summary:
Adding some boilerplate.

Create a separate diff to potentially discuss using a Tunnelbroker connection via context.

Why this solution?

1. It allows messages to be sent from any component in the tree.
2. It enables the addition of listeners from any component in the tree.
3. It facilitates dispatching actions as reactions to received messages without requiring any hacks.
4. It avoids bloating redux with messages (as in the case with the keyserver socket).

Depends on D9596

Test Plan: N/A

Reviewers: michal, bartek, tomek

Reviewed By: tomek

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9606