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/mkdocs.yml

@@ -162,6 +162,7 @@ nav:
     - Error Logs: settings/logs.md
     - Email: settings/email.md
     - Background Tasks: settings/tasks.md
+    - Currency Support: settings/currency.md
   - App:
     - InvenTree App: app/app.md
     - Connect: app/connect.md
@@ -202,6 +203,7 @@ nav:
       - API Mixin: extend/plugins/api.md
       - App Mixin: extend/plugins/app.md
       - Barcode Mixin: extend/plugins/barcode.md
+      - Currency Mixin: extend/plugins/currency.md
       - Event Mixin: extend/plugins/event.md
       - Label Printing Mixin: extend/plugins/label.md
       - Locate Mixin: extend/plugins/locate.md