Initial implementation

Move auth logic out of Card API to a shared library, this is largely a lift-and-shift with some documentation and additional tests, with the code being moved out of: https://gitlab.developers.cam.ac.uk/uis/devops/iam/card-database/card-api/-/merge_requests/90.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/identity-photos-service/photo-api/-/issues/16.

apigatewayauth/tests/mocks/migrations/0001_initial.py

+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='TestModel',
+            fields=[
+                ('name', models.TextField(primary_key=True, serialize=False, verbose_name='Name')),
+                ('isAdmin', models.BooleanField(verbose_name='Is Admin')),
+                ('principal_identifier', models.TextField(verbose_name='Principal identifier')),
+            ],
+        ),
+    ]