Add Markdown support for work item creation

[Daniel Miranda de Figueiredo (DevFigueiredo)]

Summary

• add --markdown-fields to az boards work-item create
• emit multiline-field format patch operations with the Markdown format
• cover Markdown and default plain-text request payloads with unit tests

Testing

• git diff --check
• Not run: pytest is not installed in the local environment.

Closes #1473

[Daniel Miranda de Figueiredo (DevFigueiredo)]

@microsoft-github-policy-service agree