Make SequenceProducer interface visible

Author: Coder-256

src/main/java/com/github/luben/zstd/AbstractSequenceProducer.java renamed to src/main/java/com/github/luben/zstd/SequenceProducer.java

@@ -5,7 +5,7 @@
  * SequenceProducer must be provided by the user. To use, SequenceProducer should 
  * be passed to `ZstdCompressCtx.registerSequenceProducer()`.
  */
-interface SequenceProducer {
+public interface SequenceProducer {
   /**
    * Returns a function pointer of type `ZSTD_sequenceProducer_F` that will be
    * passed to zstd as the `sequenceProducer` function. The returned pointer