DNS: support X-Auth-All-Projects header in recordsets #3803

What is missing?

It would be useful to support the X-Auth-All-Projects header in recordsets.ListByZone,
recordsets.ListAll, recordsets.Create, recordsets.Update, and recordsets.Delete, allowing
admin users to manage DNS recordsets across projects.

Proposed change

Following the same optional type assertion pattern introduced in #3800 for zones.List:

• Add AllProjects bool with the h:"X-Auth-All-Projects" tag to ListOpts, CreateOpts, and UpdateOpts
• Add ListOptsHeadersBuilder interface and implement it on ListOpts to populate pager.Headers in ListByZone and ListAll
• Add RequestOptsHeadersBuilder interface and implement it on CreateOpts and UpdateOpts to populate RequestOpts.MoreHeaders in Create and Update
• Introduce DeleteOpts with AllProjects bool and a variadic opts ...DeleteOptsBuilder parameter on Delete to support the header without breaking the existing signature.

This would be very useful to have full admin DNS management using external-dns via https://github.com/inovex/external-dns-openstack-webhook