Conduct a review of existing signaling channel implementations, research, and usage for the purpose of designing a signaling channel library API. This review will document the following areas: - Existing usage of signaling channels by applications and transports - Existing APIs of signaling channel implementations - Research on unified signaling channel libraries This is activity 1.1 of [Project 201](https://gitlab.torproject.org/tpo/team/-/wikis/projects/project-201).