spec: Add `content-type` header

[Subin Kim]

How about specifying the content-type of the webhook request in the spec?

I don't have a definite opinion on this, just a suggestion.

• content-type: application/json
  • It is the most common and standard value.
• content-type: application/vnd.standard-webhooks+json (such as application/vnd.go.cd.v1+json, application/vnd.github+json)
  • It follows vendor-specific MIME types.
  • Users can clearly understand that the webhook requests respect the standard-webhooks.
  • It is effective for gradually opt-in or switching from existing webhook consumer implementations. Through the content-type header, user can check whether it is a standard-webhook request and can perform additional authentication logic.
  • Some candidates:
    1. application/vnd.standard-webhooks+json
    2. application/vnd.standard-webhooks.v1+json
    3. application/vnd.standardwebhooks+json
    4. application/vnd.standardwebhooks.v1+json

[Vìncent Le Goff]

I'm personnaly in favor of application/vnd.standard-webhooks.v1+json
If we introduce this; should the consumer refuse anything that doesn't use this mime type then?