v4: provide wrapper for v3 backends

Hi!

As discussed on mastodon, I thought it would be a good idea to provide a translation layer (wrapper) for backends of the v3 client.

This was an issue in the past, during the v2->v3 migration: you had to have two backends in order to smoothly transition from full-v2 to full-v3 usage.

I propose, if possible, to provide a module that allows wrapping any v3 backend (maybe with a limitation of no streaming, if that helps) and get a v4 backend back. That way, there will only be one underlying backend implementation to carry around, and wrapping can happen at the use site instead of when bootstrapping the application.

Note: This is a bystander suggestion, I'm not actively using sttp these days, and probably won't be affected by the migration. This may impact my judgement on how useful/easy to implement this is ;)

Start by comparing the v3 and v4 backend interfaces and reviewing the v2-to-v3 migration context described in the issue. Determine whether a wrapper can cover v3 backends, including the proposed no-streaming limitation; done means the compatibility scope and expected behavior are clearly defined.