Add option for incremental Backups

[dreadkopp]

Is your feature request related to a problem? Please describe.
When checking the agents code for MySQL backup logic I found that it uses plain mysqldump. While this works fine, for large databases this both increases the {backup,restore}-time as well as the storage needs.

Describe the solution you'd like
At least for mariadb there is mariabackup which provides the option du run [small and fast] incremental backups. It would be great to see that functionality in portabase. documentation

Describe alternatives you've considered
Currently we use some bash scripts + cronjobs + webhooks to create hourly incremental snapshots with a full backup every 7 days. While this works fine, the options portabase offers ( web ui, various notification endpoints, overall ease-of-use ) are tempting but due to the limitations described in the intro not usable for us.

Additional context
Besides the storage requirements the 'full dump' approach might introduce issues when sending the backups to an on-premise storage (cloud>on-premise) due to bandwidth limitations. Incremental backups with much smaller size would help with that