Support string content_type in the output response

[lizzzcai]

Follow up of this issue to track the support of str content_type in output response.

expected example:

curl -i -X POST -H "Content-Type: application/json" "http://localhost:8008/v2/models/${MODEL_NAME}/infer" -d '{"inputs": [{ "name": "text", "shape": [2], "datatype": "BYTES", "data": ["I loved this food, it was very good", "I don't loved this food, it was not good"] }] }'

{"model_name":"custom-predictor__ksp-89efd40320","model_version":"v0.1.0","outputs":[{"name":"predictions","datatype":"BYTES","shape":[2],"parameters":{"content_type":{"ParameterChoice":{"StringParam":"str"}}},"data":["compliment","complaint"]}]}%  # currently it is ["Y29tcGxpbWVudA==","Y29tcGxhaW50"] and `content_type` is base64

Motivations:

1. align with V2 Inference Protocol
2. make life easier for users