public byte[] expand(int wantedBytes) {
        return expand(wantedBytes, EMPTY_BYTES);
    }
Derives a key with a given number of bytes. The returned key is always deterministic
for a HKDF initialized with a specific salt+IKM pair.

<p>If more than one key is to be derived, use {@link #expand(int, byte[])}</p>

<p>Thread safety: multiple threads can safely call <code>expand()</code> simultaneously
on the same HKDF object.</p>

@param wantedBytes Positive number of output bytes. Must be less than or equal to {@link #MAX_OUTPUT_SIZE}
@return A byte buffer of size wantedBytes filled with derived key material
security-utils/src/main/java/com/yahoo/security/HKDF.java