Add support for reporting backup events

[kaklakariada]

For administrators it would be helpful to create CloudWatch alerts that indicate if an Exasol backup succeeded or failed.

For that we can add a Metric based on events in the EXA_SYSTEM_EVENTS table. We need to report the following events:

• BACKUP_START
• BACKUP_END
• BACKUP_ABORTED

The user must be able to configure which events should be used.

[kaklakariada]

We will add the new metrics to the example dashboard in https://github.com/exasol/cloudwatch-dashboard-examples/issues/11