privileged metadata access for cross-project ACL

[Raphaël Droz]

https://docs.openstack.org/swift/latest/overview_acl.html#container-acls
states that "a container ACL does not allow access to privileged metadata (such as X-Container-Sync-Key)."

This is understandable since sensitive things are stored using "privileged" metadata (including ACL themselves IIUC).

But one useful use of "privileged metadata" is "Temp-Url-Key". The fact that we can use them using an application credentials (or any means relying upon container ACL) is really limiting.

I strongly suggest that cross-project ACL read-access may **conditionally** allow for **some** privileged metadata (Temp-Url-Key" in mind).

https://docs.openstack.org/swift/queens/middleware.html#account-acls mentions that "_Users with admin access are swift_owners and can perform any action, including viewing/setting privileged metadata (e.g. changing account ACLs)._"

An intermediary permission between "admin" and "read" would be adequate here: Allow some privileged metadata to be available as part of the read access.