Add support for explicit Shallow Cloning

[samusaran]

Add support for Shallow Cloning

Description

The current behaviour of Mapperly when mapping between same types is either to return the original instance, or to perform a deep clone of the entire object graph.

Another use case lies in the middle of those two solutions: Shallow Cloning.

Shallow cloning an object allows a user to clone an object at the first level, while keeping all the references to other reference types unchanged. If a user needs some references to be handled differently, they can still use a user mapping on those specific properties.

This feature also aligns to the default behaviour of Automapper, and this could help users to more easily migrate from Automapper to Mapperly.

Fixes #2113

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[samusaran]

@latonz I would continue the discussion here.

As stated in the github issue #2117, another solution would be to deprecate UseShallowCloning in favour of a single CloningBehaviour enum, to avoid having conflicting configuration between shallow and deep cloning.

At least from a user perspective, I find this solution to be less confusing.