Add available variables dropdown (#7964)

- Add variable dropdown
- Insert variables on click
- Save variable as `{{stepName.object.myVar}}` and display only `myVar`


https://github.com/user-attachments/assets/9b49e32c-15e6-4b64-9901-0e63664bc3e8

packages/twenty-front/src/modules/workflow/search-variables/constants/SearchVariablesDropdownId.ts

@@ -0,0 +1 @@
+export const SEARCH_VARIABLES_DROPDOWN_ID = 'search-variables';