Exchange rate plugin (#5667)

* Add plugin mixin class for supporting exchange rates

* Split some mixin classes out into their own files

- mixins.py is becoming quite bloated!

* Add some new settings for controlling currency updates

* Adds basic plugin implementation

* Refactor existing implementation

- Builtin plugin uses frankfurter.app API
- Better error / edge case handlign

* Add sample plugin for currency exchange

* Allow user to select which plugin to use for plugin updates

* Observe user-configured setting for how often exchange rates are updated

* Updates for some of the sample plugins

* Fix plugin slug

* Add doc page

* Document simple example

* Improve sample

* Add blank page for currency settings info

* More info in "config" page

* Update docs again

* Updated unit tests

* Fill out default settings values when InvenTree runs

* Add log messages

* Significant improvement in default settings speed

- Use bulk create
- Be efficient
- Dont' be inefficient

* More strict checks

* Refactor default values implementation

- Don't run at startup
- Run on list API
- Implement generic @classmethod

docs/docs/extend/plugins.md

@@ -95,6 +95,7 @@ Supported mixin classes are:
 | [APICallMixin](./plugins/api.md) | Perform calls to external APIs |
 | [AppMixin](./plugins/app.md) | Integrate additional database tables |
 | [BarcodeMixin](./plugins/barcode.md) | Support custom barcode actions |
+| [CurrencyExchangeMixin](./plugins/currency.md) | Custom interfaces for currency exchange rates |
 | [EventMixin](./plugins/event.md) | Respond to events |
 | [LabelPrintingMixin](./plugins/label.md) | Custom label printing support |
 | [LocateMixin](./plugins/locate.md) | Locate and identify stock items |

docs/docs/extend/plugins/currency.md

@@ -0,0 +1,45 @@
+---
+title: Currency Exchange Mixin
+---
+
+## CurrencyExchangeMixin
+
+The `CurrencyExchangeMixin` class enabled plugins to provide custom backends for updating currency exchange rate information.
+
+Any implementing classes must provide the `update_exchange_rates` method. A simple example is shown below (with fake data).
+
+```python
+
+from plugin import InvenTreePlugin
+from plugin.mixins import CurrencyExchangeMixin
+
+class MyFirstCurrencyExchangePlugin(CurrencyExchangeMixin, InvenTreePlugin):
+    """Sample currency exchange plugin"""
+
+    ...
+
+    def update_exchange_rates(self, base_currency: str, symbols: list[str]) -> dict:
+        """Update currency exchange rates.
+
+        This method *must* be implemented by the plugin class.
+
+        Arguments:
+            base_currency: The base currency to use for exchange rates
+            symbols: A list of currency symbols to retrieve exchange rates for
+
+        Returns:
+            A dictionary of exchange rates, or None if the update failed
+
+        Raises:
+            Can raise any exception if the update fails
+        """
+
+        rates = {
+            'base_currency': 1.00
+        }
+
+        for sym in symbols:
+            rates[sym] = random.randrange(5, 15) * 0.1
+
+        return rates
+```

docs/docs/extend/plugins/report.md

@@ -31,7 +31,7 @@ from report.models import PurchaseOrderReport
 class SampleReportPlugin(ReportMixin, InvenTreePlugin):
     """Sample plugin which provides extra context data to a report"""
 
-    NAME = "Report Plugin"
+    NAME = "Sample Report Plugin"
     SLUG = "reportexample"
     TITLE = "Sample Report Plugin"
     DESCRIPTION = "A sample plugin which provides extra context data to a report"