Add feature to automatically checkout pull request source branch

[Philipp0205]

Summary

Provide a one-click action to automatically checkout the branch associated with a pull request, allowing users to test code locally and use full IDE features.

User Story

As a developer reviewing a pull request, I want to quickly checkout the PR's source branch, so that I can run tests, debug code, and use IDE features like code navigation and refactoring.

Proposed Solution

• Add "Checkout Branch" action in pull request view
• Fetch remote branch if not present locally
• Switch to the branch using JGit/EGit APIs
• Handle uncommitted changes (stash, prompt, or warn)
• Show confirmation with branch name
• Update Eclipse workspace to reflect checked-out code

Acceptance Criteria

• "Checkout Branch" button/action available in PR detail view
• Action fetches remote branch if not available locally
• Switches active branch to PR source branch
• Handles uncommitted local changes gracefully
• Shows confirmation dialog with branch information
• Works for both GitHub and Bitbucket repositories
• Integrates with EGit branch switching UI patterns
• Updates Eclipse workspace and package explorer

Technical Considerations

• Use EGit's BranchOperationUI for branch checkout
• Fetch branch using JGit FetchCommand if not present
• Parse PR metadata to determine source branch name and remote
• Handle detached HEAD state for forked repository PRs
• Respect EGit preferences for stashing uncommitted changes
• Consider integration with EGit's "Switch To" branch dialog
• Handle authentication for private repositories