Implement as_draws generics in other packages?

(Note: I'm opening this here for discussion first rather than opening a bunch of essentially duplicate issues in several packages)

Now that posterior is on CRAN, would it make sense to have as_draws() / as_draws_XXX() implementations for objects in brms, rstanarm, rstan, cmdstanr, etc? I think it would help fulfill part of the promise of {posterior}; that is, users don't have to remember what particular function for a given model type gets them draws in what format, they can just chuck the object into as_draws_XXX() for whatever format they want.

E.g. with cmdstanr, rather than doing this:

model$draws(format = "XXX")

People could (optionally) do:

model %>% as_draws_XXX()

The advantage of the latter code is that as more packages also implement these generics it becomes easier for users to move between packages. @paul-buerkner @jgabry thoughts? I dunno if this was already the plan anyway.

Start by comparing posterior's existing as_draws/as_draws_XXX generics with conversion APIs in brms, rstanarm, rstan, and cmdstanr. Define which object classes and draw formats each integration should support, then confirm the proposal with maintainers before implementation; done means an agreed cross-package interface.