🔒️(backend) restrict resource server views

We don't want every Service Provider to be able to request
every endpoint if those are not implementing a filtering on
the data returned. To prevent any data leak we enforce the
developers to manually "whitelist" each endpoint and add
the proper filtering when needed.

src/backend/core/resource_server/mixins.py

@@ -0,0 +1,53 @@
+"""
+Mixins for resource server views.
+"""
+
+from rest_framework import exceptions as drf_exceptions
+
+from .authentication import ResourceServerAuthentication
+
+
+class ResourceServerMixin:
+    """
+    Mixin for resource server views:
+     - Restrict the authentication class to ResourceServerAuthentication.
+     - Adds the Service Provider ID to the serializer context.
+     - Fetch the Service Provider ID from the OIDC introspected token stored
+       in the request.
+
+    This Mixin *must* be used in every view that should act as a resource server.
+    """
+
+    authentication_classes = [ResourceServerAuthentication]
+
+    def get_serializer_context(self):
+        """Extra context provided to the serializer class."""
+        context = super().get_serializer_context()
+
+        # When used as a resource server, we need to know the audience to automatically:
+        # - add the Service Provider to the team "scope" on creation
+        context["from_service_provider_audience"] = (
+            self._get_service_provider_audience()
+        )
+
+        return context
+
+    def _get_service_provider_audience(self):
+        """Return the audience of the Service Provider from the OIDC introspected token."""
+        if not isinstance(
+            self.request.successful_authenticator, ResourceServerAuthentication
+        ):
+            # We could check request.resource_server_token_audience here, but it's
+            # more explicit to check the authenticator type and assert the attribute
+            # existence.
+            return None
+
+        # When used as a resource server, the request has a token audience
+        service_provider_audience = self.request.resource_server_token_audience
+
+        if not service_provider_audience:  # should not happen
+            raise drf_exceptions.AuthenticationFailed(
+                "Resource server token audience not found in request"
+            )
+
+        return service_provider_audience