Update HttpHeader schema

• add constraint: name, mapping_id and apply_to combinaison are unique
• value can be null and limit is 256 char now

src/netbox_rps_plugin/migrations/0003_http_header_unique_constraint.py

+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('netbox_rps_plugin', '0002_http_header')
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='httpheader',
+            name='value',
+            field=models.CharField(max_length=256, null=True, blank=True),
+        ),
+        migrations.AddConstraint(
+            model_name='httpheader',
+            constraint=models.UniqueConstraint(fields=('mapping', 'name', 'apply_to'), name='netbox_rps_plugin_httpheader_unique_mapping_name_apply_to'),
+        ),
+    ]