@callback broadcast(adapter_meta(), topic :: String.t(), message :: term()) ::
  :ok | {:error, term()}

Broadcasts message on given topic.

• topic - The topic to broadcast to, ie: "users:123"
• message - The payload of the broadcast

@callback child_spec(application(), config :: Keyword.t()) ::
  {:ok, [:supervisor.child_spec() | {module(), term()} | module()],
   adapter_meta()}

Return an optional supervisor spec for pub/sub.

@callback list(adapter_meta(), topic :: String.t()) :: [{term(), pid()}]

List tracked PIDs for a given topic.

@callback subscribe(adapter_meta(), topic :: String.t()) :: :ok | {:error, term()}

Subscribes the caller to the PubSub adapter's topic.

@callback track(adapter_meta(), topic :: String.t(), key :: term()) ::
  :ok | {:error, term()}

Track the current process under the given topic, uniquely identified by key.