fix(ConsumerSupervisor): allow map-style child specs

[paulswartz]

Supervisor.Spec.spec() is the older tuple-style child spec. ConsumerSupervisor.init/2 support the newer map-style child specs, but the type did not not reflect that. That resulted in Dialyzer errors like this:

example.ex:8:no_return
Function start_link/1 has no local return.
________________________________________________________________________________
example.ex:24:call
The function call will not succeed.

ConsumerSupervisor.start_link(
  _children :: [
    %{:id => _, :restart => :temporary, :start => {_, :start_link, [Keyword.t(), ...]}},
    ...
  ],
  _supervisor_opts :: [{atom(), _}, ...]
)

breaks the contract
Contract head:
([Supervisor.Spec.spec()], [option()]) :: Supervisor.on_start()

Contract head:
(module(), any()) :: Supervisor.on_start()

This updates the spec to show that start_link/2 can also accept a map.

[josevalim]

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