Add CEL-based rules for dynamically generating stub data

[sudorandom]

Changes

Resolves #9

• Adds an "--only-stubs" option so fauxrpc will return an error is there is no stub registered for the method or type
• Improves the behavior of stub database: now stubs registered to a method will not just register for the output type. This could be confusing and wrong for those who reuse output types.
• Adds support for loading stubs from a path. They are defined in JSON or YAML.
• Adds stub priority 0-100, higher priority will be considered before older ones
• Adds active_if CEL expression that will trigger a stub to be used
• Adds some examples of stub files

Examples

Return a static response:

---
stubs:
- id: find-pets-by-status
  target: io.swagger.petstore.v2.PetService/FindPetsByStatus
  content:
    pets:
    - id: 1
      category:
        id: 1
        name: cat
      name: Whiskers
      photo_urls:
      - https://cataas.com/cat
      tags:
      - id: 1
        name: cute
      - id: 2
        name: kid-friendly
      status: available

Return nothing:

---
stubs:
- id: delete-pet-empty
  target: io.swagger.petstore.v2.PetService/DeletePet
  content: {}

Return a static stub if the "pet_id" is 1, otherwise return a randomly generated stub:

---
stubs:
- id: get-pets-by-id-id-1
  target: io.swagger.petstore.v2.PetService/GetPetByID
  active_if: req.pet_id == 1
  priority: 100
  content:
    id: 1
    category:
      id: 1
      name: cat
    name: Whiskers
    photo_urls:
    - https://cataas.com/cat
    tags:
    - id: 1
      name: cute
    - id: 2
      name: kid-friendly
    status: available
- id: get-pets-by-id-default
  target: io.swagger.petstore.v2.PetService/GetPetByID
  cel_content: |
    {
        'id': req.pet_id,
        'category': {'id': gen, 'name': 'gen'},
        'name': gen,
        'photo_urls': [gen, gen],
        'tags': [{'id': gen, 'name': gen}],
        'status': gen
    }

Simply return the request as the response, (works when the req and resp are the same type):

---
stubs:
- id: add-pet
  target: io.swagger.petstore.v2.PetService/AddPet
  cel_content: req