Add --default-resource-modifier-configmap server flag

Add DefaultResourceModifierConfigMap field to the server Config struct
and bind it as a CLI flag. When set, it references a ConfigMap name in
the Velero namespace containing default resource modifier rules to apply
to all restores.

Follows the existing pattern used by --backup-repository-configmap and
--repo-maintenance-job-configmap.

Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>

pkg/cmd/server/config/config.go

@@ -182,6 +182,7 @@ type Config struct {
 	ItemBlockWorkerCount                int
 	ConcurrentBackups                   int
 	GlobalBackupVolumePoliciesConfigMap string
+	DefaultResourceModifierConfigMap    string
 }
 
 func GetDefaultConfig() *Config {
@@ -282,4 +283,10 @@ func (c *Config) BindFlags(flags *pflag.FlagSet) {
 		c.GlobalBackupVolumePoliciesConfigMap,
 		"The name of a ConfigMap in the Velero install namespace holding global backup volume policies that are merged into every backup. Optional.",
 	)
+	flags.StringVar(
+		&c.DefaultResourceModifierConfigMap,
+		"default-resource-modifier-configmap",
+		c.DefaultResourceModifierConfigMap,
+		"The name of a ConfigMap in the Velero namespace containing default resource modifier rules applied to all restores. Ignored when a per-restore resource modifier is specified.",
+	)
 }