Allow users to bring their own ActivityPub IDs, independent of server domain?

This question is the flip-side of #58. Would it be possible for users to bring their own identifiers (initially, let's talk about ENS but it can extend eventually to W3C DIDs) when signing up to a new pleroma server?

I am thinking of a scenario where we could have larger servers that are less focused on the federation but more on generalized hosting for individual users who want to (a) own their identity and (b) be able to port to different providers. Think of it as "ActivityPub-As-A-Service".

I'm not an expert on ActivityPub, but it seems that the main challenge for this to work would be that servers would have to update their Webfinger/Salmon services in order to find the correct inbox URL for each user? What other challenges do you see?

I have almost zero experience with Elixir, but I'd more than willing to learn it to develop a working implementation for Pleroma.