Implement posterior::as_draws() for rstanarm objects

[jgabry]

closes #540

Adds the various as_draws_*.stanreg methods for compatibility with the posterior package

[jgabry]

@bgoodri Ok to merge? Everything is passing except on ubuntu-latest (release), but the error is error unrelated to this PR:

 ── Failure ('test_stan_glm.R:264:7'): stan_glm returns expected result for bernoulli ──
  `val` not equal to `ans`.
  1/3 mismatches
  [3] 0.879 - 0.756 == 0.123
  cauchit