BE: Incorporate custom attribute overrides into overall JSON UI schema

There exists MetadataSourcesUiDefinitionController for rendering a server-side kept JSON schema for UI screens definition as a REST resource consumed by UI layer. The task at hand here is to keep an already existing configuration for custom attribute overrides and just add the custom property Map representing these overrides into an overall UI definition Map data representing JSON schema for metadata sources.

The possible implementation would just refactor the Controller's GET implementation and insert the overrides data into a Map we return, utilizing the existing overrides parsing and domain model components, etc.