feat: add GET/PUT /v3/settings server settings API

GET returns all settings sections except the deprecated
VirtualBox/VMware ones (Server.Audit privilege); secrets are
masked and Controller.jwt_secret_key is excluded entirely.
PUT applies a partial update (Server.Modify privilege): masked
or empty secrets mean unchanged, null removes the option, the
response carries the new values plus restart_required, and a
settings.updated notification is emitted. New
Server.Audit/Server.Modify privileges are seeded into the
Administrator role at table creation.

gns3server/schemas/__init__.py

@@ -56,6 +56,7 @@ except ImportError:
     pass
 
 from .controller.rbac import RoleCreate, RoleUpdate, Role, Privilege, ACECreate, ACEUpdate, ACE
+from .controller.settings import SettingsResponse, SettingsUpdate, SettingsUpdateResponse
 from .controller.pools import Resource, ResourceCreate, ResourcePoolCreate, ResourcePoolUpdate, ResourcePool
 from .controller.tokens import Token, ApiKeyCreate, RefreshTokenRequest
 from .controller.snapshots import SnapshotCreate, Snapshot