Accept a list of pids from deferred allowance function

[am-kantox]

When working with pools of anonymous workers, one might want to allow all the workers from the whole pool. Again, it mostly makes sense in test environment.

Unfortunately, there is no way to call allow/5 subsequently (as we do with a straightforward, non-deferred calls) because the workers can be resolved as a whole list within the context of the deferred call only.

To satisfy the aforementioned requirement (the contrived use-case might be found here although I understand it’s not the normal way of doing it, but still, pools are widely used and the ability to allow all workers looks natural to me,) this PR proposes the function to lazy resolve the allowances to be permitted to return lists of pids.

-  @spec allow(server(), pid(), pid() | (-> pid()), key()) ::
           :ok | {:error, Error.t()}
+  @spec allow(server(), pid(), pid() | (-> pid()) | (-> [pid()]), key()) ::
           :ok | {:error, Error.t()}

[am-kantox]

For coverage, if desired, I might add another test with

fn ->
  [Process.whereis(:lazy_pid_1), :not_a_pid, Process.whereis(:lazy_pid_2)]
end,

[whatyouhide]

Yeah I can't think of a better way to do this off the top of my head. 👍

[am-kantox]

@whatyouhide everything’s applied, coverage fixed; sorry for the mess with multiple commits, squash them or something.

[whatyouhide]

Thanks @am-kantox 💟