Change the way create event-contract works

[trietsch]

As the create event-contract command is quite confusing (since it expects a json representation of the proto message for EventContract, without a wrapping eventContract), let's change the command to work as follows:

strm create event-contract "bla/example/1.3.0" --schema-ref "streammachine/demo/1.0.2" --public --definition-file /path/to/definition.json

--schema-ref and --definition-file are mandatory. --public is a bool flag to make the contract public or not. The definition.json is a JSON file (maybe also support YAML? Or the ability to read from stdin?) containing the following:

{
    "keyField": "consistentValue",
    "piiFields": {
        "consistentValue": 2,
        "someSensitiveValue": 3,
        "uniqueIdentifier": 1
    },
    "validations": [
        {
            "field": "consistentValue",
            "type": "regex",
            "value": "^.+$"
        }
    ]
}

This way, we split versioning logic and a link to the actual schema (which is metadata in this case), from the actual content (the definition).