Update SeldonMessage with jsonData

[gsunner]

fixes #434

Whats implemented

• SeldonMessage updated to use a 'jsonData' field, similar in use to 'strData' etc..
• Python wrapper support for 'jsonData' field
  • Predict function in model will receive data as a 'dict' when the 'jsonData' is used.
  • When returning a 'dict' from the predict call in model, the data is returned via the 'jsonData' field.

Whats not implemented

• 'seldon_client' support in the Python wrapper
• Support for all other language wrappers

[yu-iskw]

@gsunner Thank you for the PR. I would like to know the details about the spec.

When returning a 'dict' from the predict call in model, the data is returned via the 'jsonData' field.

Especially, if I would like to express predictions as a list of dict, what data type should predict return?

#565

## inputs
{"data": [
  {"feature1": 0.1, "feature2": 0.2},
  {"feature1": 0.3, "feature2": 0.4},
]}

## outputs
{
  "predictions": [
    {"class": 3, "probability": 0.8},
    {"class": 1, "probability": 0.95},
  ],
  "meta": {
    "tags": {
      "version": "1.0.0"
    }
  }
}

[gsunner]

@yu-iskw I will try to explain how you can achieve what you need.

The input is a json object value to the "jsonData" field.
The output is a python 'dict', so put your list of predictions as a value in a dict with the key "predictions"

Here is an example how you can achieve this.

## inputs
## Predict call example input payload
{
    "jsonData": {
        "data": [
            {
                "feature1": 0.1,
                "feature2": 0.2
            },
            {
                "feature1": 0.3,
                "feature2": 0.4
            }
        ]
    }
}

## outputs
## Predict call example output
{
    "jsonData": {
        "predictions": [
            {
                "class": 3,
                "probability": 0.8
            },
            {
                "class": 1,
                "probability": 0.95
            }
        ]
    },
    "meta": {
        "metrics": [],
        "puid": "4ul51sgl3a411a0p2chr00lg5m",
        "requestPath": {
            "classifier": "gsunner/simple-model-json-value-test:v0"
        },
        "routing": {},
        "tags": {}
    }
}

[ukclivecox]

Missing one prediction.proto to change in loadtester

git ls-tree -r master --name-only | grep prediction.proto
proto/prediction.proto
python/seldon_core/proto/prediction.proto
util/loadtester/scripts/proto/prediction.proto

[yu-iskw]

@gsunner Thank you for the example. I completely understand.

[jklaise]

We should add a test, otherwise looks good. Will need to open a separate issue to update SeldonClient once this is merged.