Moderation and user-friendly Per-message Profile Management/Multi-Identity Support

Strongly in support of this and would really hate to see it go uncommented-on. Both Matrix and Stoat already support this (MSC4144 and masquerade respectively). Webhooks are a really inconvenient solution from a usability and especially a moderation standpoint. I'd definitely recommend looking over the proposal for MSC4144 for reference and inspiration. Sable's implementation of the specification is also a good frame of reference. Either way the "via parentuser" indicator is quite effective. I'm mainly so staunchly in support of this due to the utility that functionality has for moderation-friendly plural accessibility tools (allows for blocking, prevents impersonation, and prevents ban evasion), but it also still makes more sense for bots in my experience as well.