[Feature] Support preventinitialbackfill in kibana_slo

[neiljbrookes]

Is your feature request related to a problem? Please describe.

N/A

Describe the resource you would like to have implemented.

Support for the preventinitialbackfill setting in the kibana_slo resource

Describe the solution you'd like

To be able to set settings.preventinitialbackfill : true when creating an SLO.

Describe alternatives you've considered
N/A

Additional context
N/A