Update Unstable spec with email metadata schemas

[steve-henry]

Towards https://github.com/intercom/intercom/issues/340037
Adding schemas to Unstable

• email message metadata
• email address header

Updated conversation part schema to include email message metadata

    email_address_header:
      title: Email Address Header
      type: object
      description: Contains data for an email address header for a conversation part
        that was sent as an email.
      properties:
        type:
          type: string
          description: The type of email address header
          example: from
        email_address:
          type: string
          description: The email address
          example: [email protected]
        name:
          type: string
          nullable: true
          description: The name associated with the email address
          example: Hoban Washburne
    email_message_metadata:
      title: Email Message Metadata
      type: object
      description: Contains metadata if the message was sent as an email
      properties:
        subject:
          type: string
          description: The subject of the email
          example: Question about my order
        email_address_headers:
          title: Email Address Headers
          type: array
          description: A list of an email address headers.
          items:
            "$ref": "#/components/schemas/email_address_header"

[johnnynader]

Nice one 🚢