Support unpacking HeapByteBfuferR

[takaaki7]

Currently MessagePack.newDefaultUnpacker(ByteBuffer buffer) doesn't support HeapByteBuffer

When passing Read Only ByteBuffer, following exception happened. Because HeapByteBfuferR.hashArray() and HeapByteBfuferR.isDirect() returns false.

throw new IllegalArgumentException("Only the array-backed ByteBuffer or DirectBuffer is supported");

https://github.com/msgpack/msgpack-java/blob/main/msgpack-core/src/main/java/org/msgpack/core/buffer/MessageBuffer.java#L400

I want to unpacking HeapByteBufferR without copying bytes. Because I save msgpack data in BigTable, and BigTable library returns ByteString, and it only able to be no-copy converted to HeapByteBufferR with asReadOnlyByteBuffer().