INSTALL PLUGIN FORCE needed to override @@plugin_maturity

@@plugin_maturity is a readonly variable.

I believe it is reasonable to only allow mature plugins in a server, but then decide to make an exception for a specific case. A DBA is supposed to know what usage will be done, and understand the risk.

I suggest to implement a INSTALL PLUGIN FORCE syntax, that overrides @@plugin_maturity for a specific plugin. Since such syntax could be used improperly by tools that run as root, I also suggest to write a warning in the error log whenever @@plugin_maturity is overridden - hopefully, the diligent DBA monitors that file and will check if she wants that plugin.

Please keep in mind that currently, if a DBA wants to achieve the same, has to restart the server twice: to set a lower plugin_maturity and install the plugin, and then to restore the old plugin maturity.