spec: PUT /v2/<name>/manifests/<reference> MUST include Content-Type

[waynr]

This came up in discussion on the CNCF slack's #distribution channel:

• https://cloud-native.slack.com/archives/C01GVR8SY4R/p1696165659527899?thread_ts=1696017119.330229&cid=C01GVR8SY4R

The basic issue is that there is an ambiguity in the spec as-written where it's not clear what the behavior should be if neither the manifest includes a mediaType field (since that is optional) nor the PUT request includes a content-type header.

With this change from SHOULD to MUST it's clearer that registries should return an error if there is no Content-Type header.

If there are registries out there that don't enforce this and consequently there are potential clients that don't include it already then it shouldn't be difficult for those clients to issue a bug fix if they really need to be used to push to a more strict registry.