feat: ✨ Implement azdo pipelines variable-group create command

[tmeckel]

Variable Groups Management Enhancement

This branch introduces a new azdo pipelines variable-group create subcommand to enable variable group creation via CLI, along with supporting infrastructure for secure input handling and improved debugging capabilities.

New Features

Variable Groups Management

• Add azdo pipelines variable-group create subcommand
  • Create variable groups with configurable name, description, and variables
  • Support for both plaintext and secret variables
  • JSON and table output formats for consistent CLI experience

Secure Input Handling

• Implement Secret method for hidden input prompts
  • Add secure password/secret input capability to prompter interface
  • Support masked input in interactive CLI sessions
  • Maintain backward compatibility with existing prompt methods

Testing & Infrastructure

• Enhance test infrastructure
  • Add Secret method to fakePrompter for comprehensive testing
  • Extend mock prompter implementations to support secret input scenarios
  • Add test coverage for secure input handling in comment tests

Debugging & Observability

• Improve endpoint lookup debugging
  • Add debug logging for service endpoint resolution
  • Include failed endpoints in search results for better troubleshooting
  • Maintain existing error handling while adding visibility

Documentation

• Add command reference documentation
  • Generate comprehensive usage documentation for new create subcommand
  • Include examples and output format specifications
  • Maintain consistency with existing command documentation

Closes: #119