Refactoring `BaseContainerRegistry._scan_tag()`

Follow-up to #2620.

## Motivation

- The logic for handling `media_type` in `_scan_tag()` is currently defined in both `harbor.py` and `base.py`. Let's redesign the abstraction layer for this and untangle the twisted inheritance structure..

## Objective  

- Clearly distinguish between abstract methods that must be overridden and methods that do not need to be overridden. Methods that must be overridden should be marked as `@abstractmethod` in the base class.

## Details  

TODO: Write this.

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## Impact  

- Since this is a refactoring task, there should be no changes to functionality.

JIRA Issue: BA-474