[Request] Allow to init BigInt with signum and byte array

Like the JVM BigInteger, it would be greate to support this constructor. JVM doc:
```
/**
* Translates the sign-magnitude representation of a BigInteger into a
* BigInteger. The sign is represented as an integer signum value: -1 for
* negative, 0 for zero, or 1 for positive. The magnitude is a byte array
* in big-endian byte-order: the most significant byte is the
* zeroth element. A zero-length magnitude array is permissible, and will
* result in a BigInteger value of 0, whether signum is -1, 0 or 1. The
* {@code magnitude} array is assumed to be unchanged for the duration of
* the constructor call.
*
* @param signum signum of the number (-1 for negative, 0 for zero, 1
* for positive).
* @param magnitude big-endian binary representation of the magnitude of
* the number.
* @throws NumberFormatException {@code signum} is not one of the three
* legal values (-1, 0, and 1), or {@code signum} is 0 and
* {@code magnitude} contains one or more non-zero bytes.
*/
public BigInteger(int signum, byte[] magnitude) {
this(signum, magnitude, 0, magnitude.length);
}
```

Locate the BigInt implementation and its existing constructors, then compare the requested API with the JVM BigInteger documentation included here. Done means supporting signum and big-endian magnitude inputs, including zero-length magnitudes and the documented invalid-value cases; run the repository's relevant tests afterward.

Written by the indexing model from the issue text.