[Pitch] Custom metadata attributes for types, and runtime access to custom sections

This revises my pitch from last week, Runtime enumeration of custom sections, after the feedback there: it says what I want and how I would want it to work, rather than what a library can do today, and it drops the observation API. It also takes up SE-0385, which the Language Workgroup returned in 2023 asking for a design built on macros and constant initialization. Both now exist. Swift has no way for a library to find the types that opted into it. Every library that needs this works around th...