Extended the system table mysql.event to support creation of
system triggers (startup, shutdown, logon, logoff) and ddl triggers.
The system table mysql.event is extended with three columns
`kind`, `when`, `ddl_type`.
For the task MDEV-30645 only the column `kind` is required,
the other two columns are required for the tasks
MENT-2355, MENT-2291
but since it is better to reduce a number of times the system table
is changed and as a consequences a number of upgrades to be run,
the entire set of columns is added at once.
Type of the columns `kind` and `ddl_type` are specified as SET to allow
storing of triggers that handle several events.
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