MDEV-7394 : Making slave_skip_errors writable at runtime when slave is stopped

[Mahmoud-kh1]

Now we can update slave_skip_errors at runtime when slaves stopped

• This feature makes the slave_skip_errors system variable dynamic allowing it
  to be changed at runtime when the replication slave is stopped.
• Previously slave_skip_errors was read only at runtime and required a
  server restart to be changed.
• Runtime updates are now validated and safely rejected when the slave is
  running preventing inconsistent replication state.

Key Changes

• Added ON_CHECK handler to verify that updates are only allowed while the
  slave is stopped.
• Added ON_UPDATE handler to reinitialize the internal skip error state
  when the variable is changed.
• Added an rpl mtr test that verifies slave_skip_errors can be changed
  dynamically when the slave is stopped and verified that updates are rejected while the slave is running.

behavior now is like that :

Feature :
MDEV-7394

[CLAassistant]

All committers have signed the CLA.

[Mahmoud-kh1]

The following test cases fail because they assume that slave_skip_errors is read only which is no longer true which making their checks fail also
sys_vars.sysvars_server_notembedded
main.variables-notembedded
sys_vars.slave_skip_errors_basic