feat: additionalDescriptionWidget Added

[enesctn]

This PR allows adding any desired widget below the description text. The motivation for this change is to enable adding step bars to sequential descriptions.

Checklist

• [x] The title of my PR starts with a [Conventional Commit] prefix (fix:, feat:, docs: etc).
• [x] I have followed the [Contributor Guide] when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [ ] Yes, this PR is a breaking change.
• [x] No, this PR is not a breaking change.