format-request-map.json: fix azure-rhui definition

There was a missing `overrides` property in the `azure-rhui` request
definition, which resulted in:

```
$ ./tools/test-case-generators/generate-test-cases --distro rhel-90
--arch x86_64 --image-types azure-rhui --store /dev/null --output
/home/thozza/devel/osbuild-composer/test/data/manifests
--keep-image-info
Traceback (most recent call last):
  File "/home/thozza/devel/osbuild-composer/./tools/test-case-generators/generate-test-cases", line 176, in <module>
    main(args.distro, args.arch, args.image_types, args.keep_image_info, args.store, args.output)
  File "/home/thozza/devel/osbuild-composer/./tools/test-case-generators/generate-test-cases", line 153, in main
    if distro in test_case_request["overrides"]:
KeyError: 'overrides'
```

tools/test-case-generators/format-request-map.json

@@ -19,6 +19,7 @@
       "filename": "disk.vhd.xz",
       "blueprint": {}
     },
+    "overrides": {},
     "supported_arches": [
       "x86_64"
     ]