Add optional preventInitialBackfill for SLO API

Docs for this option are here: https://www.elastic.co/docs/api/doc/serverless/operation/operation-createsloop#operation-createsloop-body-application-json-settings-preventinitialbackfill

Author: jasonrhodes

internal/kibana/slo.go

@@ -457,6 +457,11 @@ func getSchema() map[string]*schema.Schema {
 						Optional: true,
 						Computed: true,
 					},
+					"preventInitialBackfill": {
+						Description: 	"Prevents the underlying ES transform from attempting to backfill data on start, which can sometimes be resource-intensive or time-consuming and unnecessary",
+						Type: 		schema.BoolAttribute,
+						Optional: 	true,
+					}
 				},
 			},
 		},