Add Markdown output support

[coderabbitai]

This issue tracks the addition of Markdown output support to httpx.

Related PR: #2344

Overview:

• Introduces a MarkDownOutput flag and CLI options -markdown / -md
• Implements Result.MarkdownOutput to generate formatted Markdown tables
• Adds markdown-escaping helper function
• Updates runner to create .md output files

Requested by: @dogancanbakir