net: improve encapsulation of CNetAddr

Summary:
```
Do not access `CNetAddr::ip` directly from `CService` methods.

This improvement will help later when we change the type of
`CNetAddr::ip` (in the BIP155 implementation).
```

Backport of core [[bitcoin/bitcoin#19360 | PR19360]].

Depends on D9169.

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9171

src/netaddress.cpp

@@ -755,14 +755,12 @@ bool CService::GetSockAddr(struct sockaddr *paddr, socklen_t *addrlen) const {
  * @returns An identifier unique to this service's address and port number.
  */
 std::vector<uint8_t> CService::GetKey() const {
-    std::vector<uint8_t> vKey;
-    vKey.resize(18);
-    memcpy(vKey.data(), ip, 16);
+    auto key = GetAddrBytes();
     // most significant byte of our port
-    vKey[16] = port / 0x100;
+    key.push_back(port / 0x100);
     // least significant byte of our port
-    vKey[17] = port & 0x0FF;
-    return vKey;
+    key.push_back(port & 0x0FF);
+    return key;
 }
 
 std::string CService::ToStringPort() const {

src/netaddress.h

@@ -193,7 +193,8 @@ class CService : public CNetAddr {
     explicit CService(const struct sockaddr_in6 &addr);
 
     SERIALIZE_METHODS(CService, obj) {
-        READWRITE(obj.ip, Using<BigEndianFormatter<2>>(obj.port));
+        READWRITEAS(CNetAddr, obj);
+        READWRITE(Using<BigEndianFormatter<2>>(obj.port));
     }
 };