Adds metadata mixin class for adding JSON field to internal tables

- Add metadata field to stocklocation
- Add metadata field to stockitem

InvenTree/stock/migrations/0075_auto_20220515_1440.py

@@ -0,0 +1,27 @@
+# Generated by Django 3.2.13 on 2022-05-15 14:40
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('stock', '0074_alter_stockitem_batch'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='stockitem',
+            name='metadata',
+            field=models.JSONField(blank=True, help_text='JSON metadata field, for use by external plugins', null=True, verbose_name='Plugin Metadata'),
+        ),
+        migrations.AddField(
+            model_name='stocklocation',
+            name='metadata',
+            field=models.JSONField(blank=True, help_text='JSON metadata field, for use by external plugins', null=True, verbose_name='Plugin Metadata'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='stocklocation',
+            unique_together=set(),
+        ),
+    ]