Add Issue/PR label management cmdlets (Add-GitHubIssueLabel, Remove-GitHubIssueLabel) #655

Summary

The module has no cmdlets for managing labels on issues or pull requests. Scripts that need to add labels to a newly-created PR must call \Invoke-GitHubAPI\ directly.

Use cases observed in the wild

• PSModule/Distributor uses \Invoke-GitHubAPI -Method POST -ApiEndpoint /repos/.../issues/{number}/labels\ to add a \NoRelease\ label after PR creation
• MSXOrg/Custo has the same pattern

Proposed cmdlets

\\powershell
Add-GitHubIssueLabel -Owner -Repository -IssueNumber -Labels <string[]> [-Context ]
Remove-GitHubIssueLabel -Owner -Repository -IssueNumber -Label [-Context ]
Get-GitHubIssueLabel -Owner -Repository -IssueNumber [-Context ]
\\

GitHub's API treats PRs as issues for label operations, so these would cover both.