Add `inference_config` to Elasticsearch model configurations

[benwtrent]

In Elasticsearch version 7.8.0, a new required field is added to trained_model_config objects. elastic/elasticsearch#54421

When Eland starts integrating against 7.8.0, it will need to include the appropriate inference config in the model payload.

eland/eland/ml/imported_ml_model.py

Line 168 in 29af761

"compressed_definition": serialized_model,

It should be added here. It might be as simple as adding the line:

"inference_config": {self._model_type: {}},

[sethmlarson]

An issue I ran into here is that if we unconditionally add the "inference_config": {self._model_type: {}}, then we run into issues with ES 7.6 where this field didn't exist.