Feature: Define Artifact and Validator contracts

[kokokuo]

Description

Define the contract of artifact type and IValidator interface for extending our built-in other validator and make the client could use it.

The artifact type from schema.yaml format:

# The name used for graphQL
name: user
url: /user/:id
request:
  parameters:
    id:
      in: query # path / query / header
      description: user id
      validators:
        - name: Date
          args:
            format: 'yyyy-MM-dd'
        - name: required
error:
  - code: Forbidden
    message: 'You are not allowed to access this resource'

[oscar60310]

LGTM