Invoicing: Allow plugins to add data (#5452)

* Allow plugins to add data to invoices

* Add documentation

src/pretix/base/migrations/0290_invoice_plugin_data.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.17 on 2025-09-09 09:55
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("pretixbase", "0289_invoiceline_period"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="invoice",
+            name="plugin_data",
+            field=models.JSONField(default=dict),
+        ),
+    ]