Closes #13381: Enable plugins to register custom data backends (#14095)

* Initial work on #13381

* Fix backend type display in table column

* Fix data source type choices during bulk edit

* Misc cleanup

* Move backend utils from core app to netbox

* Move backend type validation from serializer to model

docs/plugins/development/data-backends.md

@@ -0,0 +1,23 @@
+# Data Backends
+
+[Data sources](../../models/core/datasource.md) can be defined to reference data which exists on systems of record outside NetBox, such as a git repository or Amazon S3 bucket. Plugins can register their own backend classes to introduce support for additional resource types. This is done by subclassing NetBox's `DataBackend` class.
+
+```python title="data_backends.py"
+from netbox.data_backends import DataBackend
+
+class MyDataBackend(DataBackend):
+    name = 'mybackend'
+    label = 'My Backend'
+    ...
+```
+
+To register one or more data backends with NetBox, define a list named `backends` at the end of this file:
+
+```python title="data_backends.py"
+backends = [MyDataBackend]
+```
+
+!!! tip
+    The path to the list of search indexes can be modified by setting `data_backends` in the PluginConfig instance.
+
+::: core.data_backends.DataBackend