Add plugins for reload notifications

[MarcosDY]

In current implementation we supports sending signals or running an script in order to communicate
process that a SVID was rotated, but now we have windows where signals is not an option..
I was thinking about this and we may be able to add plugins that are used to send notifications to process (for example postgres or mysql) about there is a new SVID and they may rotate.

For now I was thinking in:

• SignalNotiication: Send a signal to an specific process
• PostgresNotification: Notify notify postgres to reload certificates
• MySqlNotification: Notify MySQL to reload certificates
• BashNotification: Run a bash script that can be used to apply changes or notify processes

The advantage is that we may be able to support different applications without depending only on signals or bash scripts.