Provide an ability to define custom bound checking for UI element interaction

## What problem does this solve or what need does it fill?

Sometimes UI elements might not be rectangular, for example - hexagonal. In such case it would be beneficial if there would be a possibility to have a custom bound checking function to be defined, so that such UI elements could work within the scope of general `ui_focus_system`.

This also includes handling events like `hover` on UI elements with different styling like rotation.

## What solution would you like?

Perhaps, we could provide a component that would contain bound-checking function. This would work for cases where bound checking does not require external information and could be calculated based on `transform` and/or `position`.

No idea how it could be implemented for more generic/complicated bound checking functions that rely on some game state.

## What alternative(s) have you considered?

Another solution would be marking "special" UI elements to be exempt from being handled by internal `ui_focus_system`.
And instead delegate responsibility for doing interactivity in such cases to developers.

Howevers, this approach makes it questionable on how to handle focus policy accross UI.

## Additional context

#5179 - is likely connected issue with this one (they originated from the same conversation).