feat: implement the scheduler plugin

This also performs a one-time migration of the DOKKU_SCHEDULER values the scheduler plugin properties.

Closes #4739

plugins/scheduler/report.go

@@ -0,0 +1,45 @@
+package scheduler
+
+import (
+	"github.com/dokku/dokku/plugins/common"
+)
+
+// ReportSingleApp is an internal function that displays the scheduler report for one or more apps
+func ReportSingleApp(appName string, format string, infoFlag string) error {
+	if err := common.VerifyAppName(appName); err != nil {
+		return err
+	}
+
+	flags := map[string]common.ReportFunc{
+		"--scheduler-computed-selected": reportComputedSelected,
+		"--scheduler-global-selected":   reportGlobalSelected,
+		"--scheduler-selected":          reportSelected,
+	}
+
+	flagKeys := []string{}
+	for flagKey := range flags {
+		flagKeys = append(flagKeys, flagKey)
+	}
+
+	trimPrefix := false
+	uppercaseFirstCharacter := true
+	infoFlags := common.CollectReport(appName, infoFlag, flags)
+	return common.ReportSingleApp("scheduler", appName, infoFlag, infoFlags, flagKeys, format, trimPrefix, uppercaseFirstCharacter)
+}
+
+func reportComputedSelected(appName string) string {
+	value := reportSelected(appName)
+	if value == "" {
+		value = reportGlobalSelected(appName)
+	}
+
+	return value
+}
+
+func reportGlobalSelected(appName string) string {
+	return common.PropertyGetDefault("scheduler", "--global", "selected", "docker-local")
+}
+
+func reportSelected(appName string) string {
+	return common.PropertyGet("scheduler", appName, "selected")
+}