Create and implement a NERModelHandler class that establishes a common method to process NER pipelines in Spark NLP

[luca-martial]

The new NERModelHandler class will include methods to:

• Wrap NER inference pipelines in Spark NLP
• Standardize output formats for all pipeline predictions

It will go under nlptest/model_handlers/

The first version of this class will support robustness testing/fixing in Spark NLP, but adaptability to other tasks and libraries should be kept in mind when developing it.