HITL - Add cross-process signal to toggle inflow of new connections.

[0mdc]

Motivation and Context

This changeset adds a cross-process signal to allow the main thread to signal whether the networking thread should accept new connections.

In the rearrange_v2 app, connections are only accepted when the application is in the "lobby" state. Note that this state is not yet merged to main.

How Has This Been Tested

Tested on multiplayer HITL app.

Types of changes

• [Development]

Checklist

• [x] My code follows the code style of this project.
• [x] I have updated the documentation if required.
• [x] I have read the CONTRIBUTING document.
• [x] I have completed my CLA (see CONTRIBUTING)
• [x] I have added tests to cover my changes if required.