First-class application/x-www-form-urlencoded support

[LucasPickering]

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Similar to #243, we should support the !form body type for Content-Type: application/x-www-form-urlencoded

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requests:
  example: !request
    method: POST
    url: "http://www.example.com/"
    headers:
      Accept: application/json
      Content-Type: application/x-www-form-urlencoded
    body: !form
      grant_type: authorization_code
      code: "{{chains.authorization_code}}"

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