net: add CServiceHash constructor so the caller can provide the salts

This new constructor will be useful if we just want to hash a `CService`
object without the two `GetRand()` calls (in `RelayAddress()` in a
subsequent commit).

src/netaddress.h

@@ -562,6 +562,14 @@ public:
 class CServiceHash
 {
 public:
+    CServiceHash()
+        : m_salt_k0{GetRand(std::numeric_limits<uint64_t>::max())},
+          m_salt_k1{GetRand(std::numeric_limits<uint64_t>::max())}
+    {
+    }
+
+    CServiceHash(uint64_t salt_k0, uint64_t salt_k1) : m_salt_k0{salt_k0}, m_salt_k1{salt_k1} {}
+
     size_t operator()(const CService& a) const noexcept
     {
         CSipHasher hasher(m_salt_k0, m_salt_k1);
@@ -572,8 +580,8 @@ public:
     }
 
 private:
-    const uint64_t m_salt_k0 = GetRand(std::numeric_limits<uint64_t>::max());
-    const uint64_t m_salt_k1 = GetRand(std::numeric_limits<uint64_t>::max());
+    const uint64_t m_salt_k0;
+    const uint64_t m_salt_k1;
 };
 
 #endif // BITCOIN_NETADDRESS_H

src/test/fuzz/netaddress.cpp

@@ -84,6 +84,8 @@ FUZZ_TARGET(netaddress)
     (void)service.ToString();
     (void)service.ToStringIPPort();
     (void)service.ToStringPort();
+    (void)CServiceHash()(service);
+    (void)CServiceHash(0, 0)(service);
 
     const CNetAddr other_net_addr = ConsumeNetAddr(fuzzed_data_provider);
     (void)net_addr.GetReachabilityFrom(&other_net_addr);