[Feature]: Signer network in-process implementation

[netrome]

Feature - Signer network in-process implementation

1. Description

Implement an in-process implementation of the signer network usable for unit tests.

1.1 Context & Purpose

To allow signer operations to be easily unit-testable, we should have an implementation of the signer network interface which operates entirely in-process without any I/O. This implementation should allow unit tests to broadcast messages between thread boundaries, and support interfering with the network process such as delaying the delivery of a message or blocking a message from being delivered. It should also support making assertions over messages being sent and received.

2.1 Acceptance Criteria:

• There is an implementation of the Signer network which runs entirely in-process.
• The implementation supports delaying or blocking messages.
• The implementation allows inspection of sent and received messages for different consumers.