add secrets field to define credential variables

[zoncoen]

The secrets field allows defining variables like the vars field. Besides, the values defined by the secrets field are masked in the outputs.

schemaVersion: scenario/v1
plugins:
  plugin: plugin.so
vars:
  clientId: abcdef
secrets:
  clientSecret: XXXXX
title: get user profile
steps:
- title: get access token
  protocol: http
  request:
    method: POST
    url: 'http://example.com/oauth/token'
    header:
      Content-Type: application/x-www-form-urlencoded
    body:
      grant_type: client_credentials
      client_id: '{{vars.clientId}}'
      client_secret: '{{secrets.clientSecret}}'
  expect:
    code: OK
    body:
      access_token: '{{$ != ""}}'
      token_type: Bearer
  bind:
    secrets:
      accessToken: '{{response.body.access_token}}'
- title: get user profile
  protocol: http
  request:
    method: GET
    url: 'http://example.com/users/zoncoen'
    header:
      Authorization: 'Bearer {{secrets.accessToken}}'
  expect:
    code: OK
    body:
      name: zoncoen

...
        --- PASS: scenarios/get-profile.yaml/get_user_profile/get_access_token (0.00s)
                request:
                  method: POST
                  url: http://example.com/oauth/token
                  header:
                    Content-Type:
                    - application/x-www-form-urlencoded
                  body:
                    client_id: abcdef
                    client_secret: {{secrets.clientSecret}}
                    grant_type: client_credentials
                response:
...
                  body:
                    access_token: {{secrets.accessToken}}
                    token_type: Bearer
                elapsed time: 0.001743 sec
        --- PASS: scenarios/get-profile.yaml/get_user_profile/get_user_profile (0.00s)
                request:
                  method: GET
                  url: http://example.com/users/zoncoen
                  header:
                    Authorization:
                    - Bearer {{secrets.accessToken}}
...

resolve #302