Add option for mysqldump to read data as of specific timestamp from system-versioned tables

It would be great to have an option for mysqldump to read data as of specific timestamp from system-versioned tables. One should just use:

select * from <t> FOR SYSTEM_TIME AS OF TIMESTAMP '...'

instead of simple SELECT for such tables if the option is provided.

I think it makes sense to implement this separately from MDEV-16029