LibCrypto: Add a constructor to (Un)SignedBigInteger taking a double

For now this will assume that the double given is exactly representable
as an integer, so no NaN, infinity or rounding.

Userland/Libraries/LibCrypto/BigInt/UnsignedBigInteger.h

@@ -39,6 +39,8 @@ public:
 
     explicit UnsignedBigInteger(u8 const* ptr, size_t length);
 
+    explicit UnsignedBigInteger(double value);
+
     UnsignedBigInteger() = default;
 
     [[nodiscard]] static UnsignedBigInteger create_invalid();