WL#5496: Plugin LOAD_OPTION in INFORMATION_SCHEMA.PLUGINS

Add a new information_schema.plugins column to show plugin load options.

There will be a new information_schema.plugins column:
LOAD_OPTION VARCHAR(64) NOT NULL

The column value is the plugin load option. Possible values: 'ON', 'OFF',
'FORCE', and (if we succeed with WL#5341 "Sticky plugins"),
'FORCE_PLUS_PERMANENT'. For example,
mysqld --audit_log=ON
causes LOAD_OPTION to contain 'ON'.

MySQL will treat automatically-loaded plugin as 'ON',
and will convert syntax with equivalent meaning
to one of the allowed values.
For example,
mysqld --disable-audit-log /* causes LOAD_OPTION to contain 'OFF' */
mysqld --audit_log=Force   /* causes LOAD_OPTION to contain 'FORCE' */

If a plugin is deactivated due to an error or due to UNINSTALL, LOAD_OPTION
value is unaffected.

See also:
dev-private email thread "WL#5341 Sticky plugins" starting with
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