Shiv Laser: Update Encapsulator to support HC_DST_MATCH BPF variant

Summary:
See D94374505 for overall context .

This diff implements C++ wrapper for functionality added in D94374505. Encapsulator class is intended to be used by other projects needing encapsulated redirection functionality.

Reviewed By: nikhildl12

Differential Revision: D94395149

fbshipit-source-id: 9faac769957ca8c5922f49362588935db8942a92

Author: avasylev

katran/lib/BalancerStructs.h

@@ -36,6 +36,17 @@ struct hc_mac {
   uint8_t mac[6];
 };
 
+// key for destination-based healthcheck matching
+struct hc_dst_key {
+  union {
+    uint32_t addr; // big-endian (__be32)
+    uint32_t addrv6[4]; // big-endian (__be32)
+  };
+  uint16_t port; // big-endian (__be16); 0 = wildcard
+  uint8_t flags; // V6DADDR if IPv6
+  uint8_t pad;
+};
+
 // vip's definition for lookup
 // also used for hc_keys
 struct vip_definition {