[Feature Request]: Ability to register translators with Flink Runners

### What would you like to happen?

Currently the implementation for FlinkRunner is in a manner that it doesn't allow adding new translators from outside of the class **FlinkStreamingTransformTranslators**. There is a private TRANSLATORS hashmap which cannot be accessed outside of the class and the class doesn't expose any method to add a translator to that hashmap. This restricts the ability to add some custom translator to Flink runner unless someone makes changes to FlinkStreamingTransformTranslators class.

Other runners has exposed some methods or Registrars for this usecase:

- Dataflow runner has **DataflowPipelineTranslator.registerTransformTranslator()**

- SamzaRunner offers **SamzaTranslatorRegistrar** which allows registering translators.

Can we have expose some methods in Flink Runner which gives this functionality or is there a particular reason Flink Runner has been implemented in such a way ?

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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