feat: add issue and epic linking functionality to update command

Description

This merge request adds relationship management functionality to the glab issue update command, allowing users to link/unlink issues and assign/remove epic associations directly from the CLI.

Key Changes:

• Add issue linking support: New --linked-issues flag accepts comma-separated issue IIDs with configurable link types via --link-type (relates_to, blocks, is_blocked_by)
• Add issue unlinking support: New --unlink-issues flag to remove existing links between issues
• Add epic assignment support: New --epic flag to assign issues to epics or remove from epics (set to 0)
• Optimized API calls: Only call UpdateIssue API when there are actual issue property changes, reducing unnecessary requests
• Enhanced user feedback: Clear progress messages and success confirmations for all relationship operations
• Validation: Validates link types and checks for existing relationships before unlinking

Implementation Details:

• Added handleIssueLinks() function to manage all linking/unlinking operations
• Added epic assignment logic directly in main update flow with proper action tracking
• Enhanced command examples with both linking and epic assignment use cases
• Added error handling for missing links, invalid link types, and epic operations
• Maintains full backward compatibility with existing update functionality

Related Issues

How has this been tested?

• Unit tests: Added test coverage for new flags and default values in issue_update_test.go
• Epic functionality tests: Added specific tests for epic flag validation and assignment/removal logic
• Manual testing: Verified linking/unlinking and epic assignment functionality with various combinations
• Integration testing: Confirmed optimized API calls work correctly - UpdateIssue only called when needed
• Error handling: Tested edge cases including invalid link types, non-existent relationships, and epic operations
• Documentation: Auto-generated documentation updated via make gen-docs to include new functionality
• Backward compatibility: Verified all existing glab issue update functionality remains unchanged

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap