DHCP: Host Name Override without static IP reservation

[Hemsby]

Summary

• Reserved Leases currently couple two separate things: forcing a client onto a static IP, and overriding its DNS hostname. There was no way to get the hostname override without also pinning the IP.
• Adds a new, independent "Host Name Override" concept (MAC-keyed, no IP attached) alongside Reserved Leases. A client with only a Host Name Override still gets a normal dynamically allocated lease; the resolved hostname is stored on that dynamic lease and flows through the existing dynamic-lease DNS registration path (including its existing same-hostname-different-address guard).
• New API endpoints addHostNameOverride / removeHostNameOverride, hostNameOverrides field on scope get/set, and a matching "Host Name Overrides" table in the scope editor UI, mirroring the existing Reserved Leases UI minus the IP column.
• Scope binary format bumped 10 -> 11 to persist the new entries; existing scope files on disk load unchanged (version-gated read).

Test plan

• Built DnsServerCore/DnsServerApp in Debug and Release with zero errors/warnings.
• Deployed to a test server, added a Host Name Override for a client, confirmed it received a normal dynamically allocated IP with the overridden hostname registered in DNS.
• Confirmed existing Reserved Lease behavior is unchanged (regression check).
• Deployed to a production DHCP server with 8 active scopes; confirmed all existing scope files loaded cleanly under the new binary format version with no errors, and the feature works end-to-end against real client traffic.