Core/Authserver: Authserver cleanup (PR#25093)

- Fix a handful of 1/256 bugs with most significant byte zero in BigNumber
- Get rid of (most of) the C-style arrays in authserver
- CryptoRandom as a unified source for cryptographic randomness
- Bring our other crypto APIs into 2020
- BigNumber usability improvements
- Authserver is now actually readable as a result of all of the above

(cherry picked from commit 210176fd91)

src/server/bnetserver/Server/Session.cpp

@@ -18,6 +18,7 @@
 #include "Session.h"
 #include "BattlenetRpcErrorCodes.h"
 #include "ByteConverter.h"
+#include "CryptoRandom.h"
 #include "DatabaseEnv.h"
 #include "Errors.h"
 #include "IPLocation.h"
@@ -386,9 +387,8 @@ uint32 Battlenet::Session::VerifyWebCredentials(std::string const& webCredential
         if (!_ipCountry.empty())
             logonResult.set_geoip_country(_ipCountry);
 
-        BigNumber k;
-        k.SetRand(8 * 64);
-        logonResult.set_session_key(k.AsByteArray(64).get(), 64);
+        std::array<uint8, 64> k = Trinity::Crypto::GetRandomBytes<64>();
+        logonResult.set_session_key(k.data(), 64);
 
         _authed = true;