Structural improvements: enhance headers handling

Following (again) ideas from OAI/OpenAPI-Specification#563 and OAI/sig-moonwalk#115, extending OAI/OpenAPI-Specification#369:

• define reusable response headers (like parameters or definitions as proposed in OAI/OpenAPI-Specification#563 by @fehguy )
• set responses headers on each level: whole api, path, operation, responses (and response as stated in OAI/OpenAPI-Specification#369)

paths:
  responseHeaders:
     # Headers returned on all api responses
  /resources:
    responseHeaders:
      # Headers returned on all /resources operations responses
    get:
      responseHeaders:
        # Headers returned on all operations responses
      200:
         headers:
           # Headers returned on this response
schemas:
  responseHeaders:
    # reusable headers describe with a Header Object, used with $ref

The problem is the naming consistency: responseHeaders almost everywhere vs headers on response level.
What if we use headers or responseHeaders everywhere?

nb: and don't forget to modify Header Object to include OAI/OpenAPI-Specification#321 (required)

リンクされている OpenAPI Specification の Issue #563、#115、#369、#321 を確認し、その後、この Issue で提案されている responseHeaders と headers の配置を比較してください。仕様を変更する前に、一貫した命名と Header Object の要件を決定してください。構造上の提案と必須ヘッダーの動作が解決され、一貫して文書化されていれば完了です。