[DeclarativePolicy] Allow overrides

What does this MR do?

Fixes #207482 (closed)

This MR addresses the inability to override parent policies by making this a declared rule. When a policy class declares that an ability is overridden, it will not be looked for in the parent ability-map, just the own one.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done