introduce Message structs

[panentheos]

Summary of changes

This introduces a new Message protocol and associated structs as part of the ongoing refactoring effort. These new structs represent a piece of sign content at a purely semantic level, in contrast with the existing Content.Message structs which encode more concrete, line-level content. The new protocol allows these abstract messages to be "rendered" into various concrete visual (and eventually audio) forms.

Conceptually, message generation is broken into several steps: First, generate the abstract messages to display. Then, optionally transform these messages, e.g. to combine headway or alert statuses. Finally, render the messages into actual line content, including flipping, shortening, or paging as needed.

One immediate benefit of this change is that the message generation algorithm is no longer split between the platform and mezzanine cases. A followup PR will leverage this new framework to generate audio messages as well.

Note that although we still assume we're dealing with at most 2 messages, nothing about this approach specifically requires that. It would be reasonably straightforward to support more messages at some point by rewriting transform_messages and render_messages accordingly.

[PaulJKim]

This is looking great to me! I'll take another look in the AM during the usual code review block but not seeing any concerns

[PaulJKim]

🧹 Looks great!