[Feature Request] Markdown support for az boards work-item create

**Is your feature request related to a problem? Please describe.**
Yes. Currently, when creating work items via the Azure DevOps CLI (az boards work-item create), Markdown formatting is not supported in large text fields such as System.Description. This is different in the Azure DevOps UI and REST API, [which do support Markdown](https://devblogs.microsoft.com/devops/markdown-support-arrives-for-work-items). As a result, users who rely on CLI workflows cannot benefit from Markdown when creating work items.

**Describe the solution you'd like**
I would like the Azure DevOps CLI to support Markdown formatting in large text fields during work item creation. Maybe adding a parameter or flag (e.g., --markdown) that allows users to specify Markdown as the format for fields like System.Description, Acceptance Criteria, or any custom multiline fields.

This would mirror the REST API functionality, where Markdown can be enabled using:
```
{ "op": "add", "path": "/fields/System.Description", "value": "# some markdown text" },
{ "op": "add", "path": "/multilineFieldsFormat/System.Description", "value": "Markdown" }
```

The CLI should internally apply the same logic to support Markdown formatting.

**Additional context**

Relevant article:
https://devblogs.microsoft.com/devops/markdown-support-arrives-for-work-items/