Introduce MapValueAttribute to map constant values and method provided values

[latonz]

MapValueAttribute

Introduces a new MapValueAttribute which allows mapping constant values and method provided values to target members.

Fixes #631

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)