Authentication proxy isn't able to prevent the zone from being deleted

[smangels]

Description

• when account is set to user-x (in PDNS) and allow_suffix_creation is empty, we're not able to prevent the ZONE from being deleted.

Proposal

• introduce a new variable in proxy.ini where permissions are described as READ, UPDATE, DELETE
• READ - read only, possible to list the zone metadata
• UPDATE - add, change and remove records
• DELETE - everything from above + delete zone