Fixed drf JSONField being interpreted as Any

Drf JSONField is actually 'any' because it can be of any type (list, obj
int, bool, etc). Nevertheless, we (and drf-spectacular) always interpret
it as being of type 'object'.

In v0.27.0, drf-spectacular fixed that on their side, which broke us.

Although it is not correct to keep using JSONFields strictly as a object,
this commit adds a drf-spectacular extension that enforces that it should
always be trated as an 'object' on openapi schema generation.

The reason is to provide a safe fix for our users, who need their
bindings working the same way as they always did.

Closes: pulp#3639

CHANGES/3639.bugfix

@@ -0,0 +1 @@
+Fixed openapi schema generation by monkeypatching that drf JSONField is always an OA 'object'.

pulp_rpm/app/__init__.py

@@ -11,3 +11,9 @@ class PulpRpmPluginAppConfig(PulpPluginAppConfig):
     version = "3.28.0.dev"
     python_package_name = "pulp-rpm"
     domain_compatible = True
+
+    def ready(self):
+        # include drf-spectacular Extension module
+        import pulp_rpm.app.schema.extensions  # noqa: E402, F401
+
+        super().ready()

pulp_rpm/app/schema/extensions.py

@@ -0,0 +1,24 @@
+"""drf-spectacular Extensions.
+
+See:
+https://drf-spectacular.readthedocs.io/en/latest/customization.html#step-5-extensions
+"""
+
+from drf_spectacular.extensions import OpenApiSerializerFieldExtension
+from drf_spectacular.plumbing import build_basic_type
+from drf_spectacular.types import OpenApiTypes
+
+
+class RestrictedJsonFieldExtension(OpenApiSerializerFieldExtension):
+    """
+    Workaround to makes drf JSONField to produce only Object Type.
+
+    See:
+    https://github.com/tfranzel/drf-spectacular/issues/1242
+    """
+
+    target_class = "rest_framework.fields.JSONField"
+    # match_subclasses = True  # not needed here but good to know.
+
+    def map_serializer_field(self, auto_schema, direction):
+        return build_basic_type(OpenApiTypes.OBJECT)