[Subscription API] add credentail type cloud in subscription spec sink credentials

[xdlbdy]

Under the subscription spec Sink Credentials the predefined credentail types are "PLAIN", "ACCESSTOKEN", and "REFRESHTOKEN". Now more and more applications are using products in the cloud. The authentication of cloud generally using accessKeyId and secretAccessKey, it is differcent with existing credentail types, i think it is necessary add a new credentail type "CLOUD"

[duglin]

I'm curious if "secretAccessKey" isn't the same thing as "ACCESSTOKEN" in this context?
But pinging @clemensv for his perspective.

[github-actions]

This issue is stale because it has been open for 30 days with no
activity. Mark as fresh by updating e.g., adding the comment /remove-lifecycle stale.