Add repository pipelines config subresources

[GrahamCampbell]

Summary

• Add pipelinesConfig($repo)->ssh() for the existing repository pipelines config SSH API.
• Add pipelinesConfig($repo)->buildNumber() and pipelinesConfig($repo)->schedules() accessors for existing subresources.
• Update the V5.1 changelog.

Notes

• Fixes Missing SSH on pipeline #93.
• Supersedes Add ssh namespace #94 by adding ssh() under pipelinesConfig(), which matches Bitbucket's /pipelines_config/ssh/... endpoints.

Testing

• php -l src/Api/Repositories/Workspaces/PipelinesConfig.php
• No tests added to stay consistent with the existing test style for this API surface.
• Existing PHPUnit/PHPStan/Psalm binaries are not installed in this checkout (vendor-bin/*/vendor/bin/* missing).