plugins lack a proper API to understand if DLQ is enabled

Currently plugins have access to the `execution_context.dlq_writer`, that can be three things:

- `nil` if Logstash does not support DLQ
- `PluginDeadLetterQueueWriter` with a `DummyDeadLetterQueueWriter` `inner_writer` if Logstash supports DLQ but DLQ isn't enabled
- `PluginDeadLetterQueueWriter` with a `DeadLetterQueueWriter` `inner_writer` if Logstash supports DLQ and DLQ is enabled

This means that if a plugin wants to know if DLQ is enabled, it's necessary to do:

`@dlq_writer.nil? || @dlq_writer.inner_writer.is_a?(DummyDeadLetterQueueWriter)`

It'd be nice if core provided a better API for plugins to query this state without knowing about the `inner_writer`