Add Pull Request management cmdlets (New-GitHubPullRequest, Get-GitHubPullRequest, Set-GitHubPullRequest)

[MariusStorhaug]

Summary

The module has no dedicated Pull Request cmdlets. Scripts that need to create or query PRs must fall back to raw \Invoke-GitHubAPI\ calls, which is verbose and loses the benefits of typed output, tab completion, and discoverability.

Use cases observed in the wild

• PSModule/Distributor uses \Invoke-GitHubAPI -Method GET -ApiEndpoint /repos/.../pulls\ to list open PRs
• PSModule/Distributor uses \Invoke-GitHubAPI -Method POST -ApiEndpoint /repos/.../pulls\ to create PRs
• MSXOrg/Custo has the same pattern

Proposed cmdlets

\\powershell
Get-GitHubPullRequest -Owner -Repository [-Head ] [-State <open|closed|all>] [-Context ]
New-GitHubPullRequest -Owner -Repository -Title -Head -Base [-Body ] [-Draft] [-Context ]
Set-GitHubPullRequest -Owner -Repository -PullNumber [-Title ] [-Body ] [-State <open|closed>] [-Context ]
\\

Output type: \GitHubPullRequest\ (new typed class, or reuse an existing one if it exists).