chore: suggest nested properties in action filters (#6533)

https://linear.app/unleash/issue/2-2029/support-filtering-on-nested-properties

Suggests nested properties in action filters. Also sorts them
alphabetically.

Follow up to https://github.com/Unleash/unleash/pull/6531

<img width="381" alt="image"
src="https://github.com/Unleash/unleash/assets/14320932/4e2c900d-335b-4360-8be4-186f3887e42b">

frontend/src/component/project/Project/ProjectSettings/ProjectActions/ProjectActionsTable/ProjectActionsModal/ProjectActionsForm/ProjectActionsFormStep/ProjectActionsFormStepSource/ProjectActionsFormStepSource.tsx

@@ -13,6 +13,7 @@ import Add from '@mui/icons-material/Add';
 import { ProjectActionsPreviewPayload } from './ProjectActionsPreviewPayload';
 import { useSignalEndpointSignals } from 'hooks/api/getters/useSignalEndpointSignals/useSignalEndpointSignals';
 import { ConditionallyRender } from 'component/common/ConditionallyRender/ConditionallyRender';
+import { flattenPayload } from '@server/util/flattenPayload';
 
 const StyledDivider = styled(Divider)(({ theme }) => ({
     margin: theme.spacing(2, 0),
@@ -86,7 +87,9 @@ export const ProjectActionsFormStepSource = ({
         const lastSourcePayload = signalEndpointSignals[0]?.payload;
         return {
             lastSourcePayload,
-            filterSuggestions: Object.keys(lastSourcePayload || {}),
+            filterSuggestions: Object.keys(
+                flattenPayload(lastSourcePayload) || {},
+            ).sort(),
         };
     }, [signalEndpointSignals]);
 

src/lib/util/flattenPayload.ts

@@ -1,8 +1,9 @@
 export const flattenPayload = (
-    payload = {},
+    payload: Record<string, unknown> = {},
     parentKey = '',
 ): Record<string, unknown> =>
-    Object.entries(payload).reduce((acc, [key, value]) => {
+    Object.entries(payload).reduce(
+        (acc, [key, value]) => {
             const newKey = parentKey ? `${parentKey}.${key}` : key;
 
             if (
@@ -11,7 +12,10 @@ export const flattenPayload = (
                 !Array.isArray(value)
             ) {
                 // If it's an object, recurse and merge the results
-            Object.assign(acc, flattenPayload(value, newKey));
+                Object.assign(
+                    acc,
+                    flattenPayload(value as Record<string, unknown>, newKey),
+                );
             } else if (Array.isArray(value)) {
                 // If it's an array, map through it and handle objects and non-objects differently
                 value.forEach((item, index) => {
@@ -29,4 +33,6 @@ export const flattenPayload = (
             }
 
             return acc;
-    }, {});
+        },
+        {} as Record<string, unknown>,
+    );