feat: support named mappings for existing target objects

[clegoz]

feat: support named mappings for existing target objects

Description

Fixes #1649
Fixes #1700

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)

[latonz]

Thanks for your contribution. Could you please explain the reason behind your choice of a different approach for implementing this compared to the new instance mappings (using UseNamedMappingBuilder.TryBuildMapping)? I'm concerned that your implementation doesn't seem to address diagnostics like RMG072 and RMG073. Could you please add tests to verify this and maybe adjust your implementation to match the existing approach (see UseNamedMappingBuilder.TryBuildMapping)?

[clegoz]

I made changes accordingly to warn when types do not match, and also added CompositeMemberExistingTargetMapping (not sure about the name) to map source and target types to the referenced mapping types.