Use model and view when describing the message type

[vordimous]

Is your feature request related to a problem? Please describe.
Describe the message type using unique naming to not conflate other aspects of the zilla config like a binding's type.

Describe the solution you'd like
Use the word model when referring to the message type as it is stored and transmitted. Use the word view when describing a the message type after the stored message is converted from or to.

generic example

key:
  model: string
value:
  model: protobuf
  view: json

Additional context
Specific examples from the bindings this change may affect.

Kafka:

type: kafka
kind: cache_server
options:
  topics:
    - name: my_topic
      value:
        model: protobuf
        view: json
        catalog:
          my_registry:
            - id: 1

HTTP:

type: http
kind: server
options:
  requests:
    - path: /hello
      method: GET
      content:
        model: json
    - path: /valid/{category}/{id}
      method: POST
      content-type:
        - application/json
      headers:
        code:
          model: string
          length: 13
      params:
        path:
          category:
            model: string
            length: 13
          id:
            model: integer
        query:
          page:
            model: integer
      content:
        model: json

MQTT

type: mqtt
kind: server
options:
  topics:
    - name: sensor/one
      content:
        model: json

Tasks

• refactor types-* modules to model-* modules
• unify ValidatorFactorySpi and ConverterFactorySpi to ModelFactorySpi
  • unify Validator and Converter to Model
  • unify ValidatorContext and ConverterContext to ModelContext
  • retain ValidatorHandler and ConverterHandler obtained from ModelContext
  • consider implications of unifying ValidatorConfig and ConverterConfig into ModelConfig, restrict view via schema?
• update http and mqtt reference docs to allow only validators (not converters) in type information (using model view terms now instead)
• document protobuf support in zilla reference (using model view terms)