feat: add fields for V2 Busway screens

[digitalcora]

Asana: https://app.asana.com/0/1185117109217413/1206932080790885

This is intended to account for the full range of configuration possible for v1 Solari screens, which is documented here. Reviewers should hold me to that 😄

Things to note:

• I've added some documentation to the modules I touched, in an attempt to start writing down our expectations of how the configuration should make signs behave. Ideally this is not too detailed (it's not an implementation guide), or there will be drift between the documentation and reality if we change how an existing field is interpreted and forget to update the docs here.

• Per discussion with @jzimbel-mbta, the pill field from the v1 config is omitted, as this information can be derived from the other fields (and DUP displays already do this, so we would not need to write the logic from scratch).

• The filter field of Section does not seem to be used in any active configuration, and was probably only added to accommodate busway screens in the future. Because of this I thought it should be okay to rename this to filters, nest the route-direction filtering inside it, and then that allows us to add the "minutes" filtering in a logical place. I'm not sure if changing the existing structure like this actually works, though (I expect it to quietly drop the old field the next time the configuration is serialized, but if that's not the case, this might need to change.)

[jzimbel-mbta]

(Just swooping in to set Christian as the assignee—we have a convention of using the assignee field to indicate whose court the ball is currently in, so to speak. I personally use https://github.com/pulls/assigned when looking for stuff to review, so if it's not set I'm likely to miss the PR! Not sure if Christian does the same thing.)

edit: lol you just did it nvm!

[cmaddox5]

Looks great! The moduledocs are very clear and answered any question I had while reviewing.