C API: Add `rocksdb_iter_is_key/value_pinned` functions

There are no C bindings to verify whether an iterator key or value are pinned. Given #14895, I'd find it useful if the C API had this:
```c
extern ROCKSDB_LIBRARY_API unsigned char rocksdb_iter_is_key_pinned(const rocksdb_iterator_t*);
extern ROCKSDB_LIBRARY_API unsigned char rocksdb_iter_is_value_pinned(const rocksdb_iterator_t*);
```