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

InvenTree/plugin/builtin/integration/currency_exchange.py

@@ -0,0 +1,53 @@
+"""Builtin plugin for requesting exchange rates from an external API."""
+
+
+import logging
+
+from django.utils.translation import gettext_lazy as _
+
+from plugin import InvenTreePlugin
+from plugin.mixins import APICallMixin, CurrencyExchangeMixin
+
+logger = logging.getLogger('inventree')
+
+
+class InvenTreeCurrencyExchange(APICallMixin, CurrencyExchangeMixin, InvenTreePlugin):
+    """Default InvenTree plugin for currency exchange rates.
+
+    Fetches exchange rate information from frankfurter.app
+    """
+
+    NAME = "InvenTreeCurrencyExchange"
+    SLUG = "inventreecurrencyexchange"
+    AUTHOR = _('InvenTree contributors')
+    TITLE = _("InvenTree Currency Exchange")
+    DESCRIPTION = _("Default currency exchange integration")
+    VERSION = "1.0.0"
+
+    def update_exchange_rates(self, base_currency: str, symbols: list[str]) -> dict:
+        """Request exchange rate data from external API"""
+
+        response = self.api_call(
+            'latest',
+            url_args={
+                'from': [base_currency],
+                'to': symbols,
+            },
+            simple_response=False
+        )
+
+        if response.status_code == 200:
+
+            rates = response.json().get('rates', {})
+            rates[base_currency] = 1.00
+
+            return rates
+
+        else:
+            logger.warning("Failed to update exchange rates from %s: Server returned status %s", self.api_url, response.status_code)
+            return None
+
+    @property
+    def api_url(self):
+        """Return the API URL for this plugin"""
+        return 'https://api.frankfurter.app'