Make innodb_flush_method dynamic

The new default value of innodb_flush_method, O_DIRECT, is not the best for some kinds of load, like queries requiring large scans (see MDEV-30986) in 10.6, while it's the best for small data changes. That is, depending on the instance role, the optimal value may be different.

Now consider the case when instance changes the role (like replica promoted to master after the crash of current master, or due to maintenance needed). It would really help to have a way to change this setting without a restart (and further warming up etc), to provide better HA, while keeping the best performance for the new role.

It will also help to speed up and simplify testing the impact of different innodb_flush_method values on the load, adapt on the fly etc. We let buffer pool to be resized without a restart, but can not control this aspect of writing it's content to disk. This is not logical, hence this request.