DNS: support X-Auth-All-Projects header in zones.List #3800

What is missing?

It would be useful to support X-Auth-All-Projects header in zones.List, allowing admin
users to enumerate DNS zones across all projects.

Proposed change

Following the same pattern as ListShares / ListSharesOptsBuilder in the same file:

• Add a ListOptsHeadersBuilder interface with a ToZoneListHeadersMap() method
• Add AllProjects bool with the h:"X-Auth-All-Projects" tag to ListOpts
• Implement ToZoneListHeadersMap() on ListOpts
• Update List() to accept an optional ListOptsHeadersBuilder and populate pager.Headers