MDEV-14293: MyRocks lacks basic functionality

The error
  "Unsupported collation on string indexed column %s Use
    binary collation (latin1_bin, binary, utf8_bin)."

is misleading. Change it:
- It is now a warning
- It is printed only for collations that do not support index-only access
  (reversible collations that use unpack_info are ok)
- The new warning text is:

  Indexed column %s.%s uses a collation that does not allow index-only
  access in secondary key and has reduced disk space efficiency
  in primary key.

storage/rocksdb/rdb_datadic.h

@@ -1358,4 +1358,6 @@ struct Rdb_index_info {
   uint64 m_ttl_duration = 0;
 };
 
+bool rdb_is_collation_supported(const my_core::CHARSET_INFO *const cs);
+
 } // namespace myrocks