Added ZSTD_get_decompressed_size

Since this implementation handles multiple concatenated frames,
to determine decompressed size we must traverse the entire input,
checking each frame's frame_content_size field

contrib/educational_decoder/zstd_decompress.h

@@ -3,4 +3,5 @@ size_t ZSTD_decompress(void *dst, size_t dst_len, const void *src,
 size_t ZSTD_decompress_with_dict(void *dst, size_t dst_len, const void *src,
                                  size_t src_len, const void *dict,
                                  size_t dict_len);
+size_t ZSTD_get_decompressed_size(const void *src, size_t src_len);