LibTLS: Add signature verification for DHE and ECDHE key exchange

This will verify that the signature of the ephemeral key used in the
DHE and ECDHE key exchanges is actually generated by the server.

This verification is done using the first certificate provided by the
server, however the validity of this certificate is not checked here.
Instead this code expects the validity to be checked earlier by
`TLSv12::handle_certificate`.

Userland/Libraries/LibCrypto/Hash/HashManager.h

@@ -200,6 +200,11 @@ public:
     }
 #endif
 
+    inline HashKind kind() const
+    {
+        return m_kind;
+    }
+
     inline bool is(HashKind kind) const
     {
         return m_kind == kind;