Remove default ctors from SecretKey and PublicKey: (#4607)

* It is now an invariant that all constructed Public Keys are valid,
  non-empty and contain 33 bytes of data.
* Additionally, the memory footprint of the PublicKey class is reduced.
  The size_ data member is declared as static.
* Distinguish and identify the PublisherList retrieved from the local
  config file, versus the ones obtained from other validators.
* Fixes #2942

src/test/protocol/PublicKey_test.cpp

@@ -363,10 +363,12 @@ public:
         }
 
         // Try some random secret keys
-        std::array<PublicKey, 32> keys;
+        std::vector<PublicKey> keys;
+        keys.reserve(32);
 
-        for (std::size_t i = 0; i != keys.size(); ++i)
-            keys[i] = derivePublicKey(keyType, randomSecretKey());
+        for (std::size_t i = 0; i != keys.capacity(); ++i)
+            keys.emplace_back(derivePublicKey(keyType, randomSecretKey()));
+        BEAST_EXPECT(keys.size() == 32);
 
         for (std::size_t i = 0; i != keys.size(); ++i)
         {
@@ -447,7 +449,11 @@ public:
         BEAST_EXPECT(pk1 == pk2);
         BEAST_EXPECT(pk2 == pk1);
 
-        PublicKey pk3;
+        PublicKey pk3 = derivePublicKey(
+            KeyType::secp256k1,
+            generateSecretKey(
+                KeyType::secp256k1, generateSeed("arbitraryPassPhrase")));
+        // Testing the copy assignment operation of PublicKey class
         pk3 = pk2;
         BEAST_EXPECT(pk3 == pk2);
         BEAST_EXPECT(pk1 == pk3);

src/test/protocol/STObject_test.cpp

@@ -17,7 +17,6 @@
 */
 //==============================================================================
 
-#include <ripple/basics/Log.h>
 #include <ripple/beast/unit_test.h>
 #include <ripple/json/json_reader.h>
 #include <ripple/json/to_string.h>
@@ -609,12 +608,7 @@ public:
     auto const kp =
         generateKeyPair(KeyType::secp256k1, generateSeed("masterpassphrase"));
     st[sf5] = kp.first;
-    BEAST_EXPECT(st[sf5] != PublicKey{});
     st[~sf5] = std::nullopt;
-#if 0
-            pk = st[sf5];
-            BEAST_EXPECT(pk.size() == 0);
-#endif
 }
 
 // By reference fields

src/test/protocol/SecretKey_test.cpp

@@ -275,10 +275,12 @@ public:
         }
 
         // Try some random secret keys
-        std::array<SecretKey, 32> keys;
+        std::vector<SecretKey> keys;
+        keys.reserve(32);
 
-        for (std::size_t i = 0; i != keys.size(); ++i)
-            keys[i] = randomSecretKey();
+        for (std::size_t i = 0; i != keys.capacity(); ++i)
+            keys.emplace_back(randomSecretKey());
+        BEAST_EXPECT(keys.size() == 32);
 
         for (std::size_t i = 0; i != keys.size(); ++i)
         {