Mention about xz and zstandard as default registerd codecs in JavaDoc

* Currently there are three codecs registered by default:
* <ul>
* <li>{@code null}</li>
* <li>{@code deflate}</li>
* <li>{@code snappy}</li>
* <li>{@code bzip2}</li>
* </ul>

static {
  addCodec(DataFileConstants.NULL_CODEC, nullCodec());
  addCodec(DataFileConstants.DEFLATE_CODEC, deflateCodec(DEFAULT_DEFLATE_LEVEL));
  addCodec(DataFileConstants.BZIP2_CODEC, bzip2Codec());
  addCodec(DataFileConstants.XZ_CODEC, xzCodec(DEFAULT_XZ_LEVEL));
  addCodec(DataFileConstants.ZSTANDARD_CODEC, zstandardCodec(DEFAULT_ZSTANDARD_LEVEL, DEFAULT_ZSTANDARD_BUFFERPOOL));
  addCodec(DataFileConstants.SNAPPY_CODEC, snappyCodec());
}