Make ExpectedImpact JSON serializable

[hungcs]

this enum not being json-serializable causes problems on the predibase side, since we return the ludwig config to the client over http

  File "/Users/hw/mambaforge/envs/based/lib/python3.8/json/encoder.py", line 179, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type ExpectedImpact is not JSON serializable.' -- invalid character 'T' looking for beginning of value

https://stackoverflow.com/questions/36699512/is-it-possible-to-dump-an-enum-in-json-without-passing-an-encoder-to-json-dumps

tested locally