pgmoneta: Storage engines

pgmoneta’s current storage layer is fragmented, back-ends do not share full feature parity and a single backup cannot target multiple destinations at once. This project introduces a unified storage interface that every backend implements, enabling easier maintenance and concurrent multi-backend writes. SSH and Azure backends are raised to the same capabilities as S3, while new native drivers for Google Cloud Storage (GCS) and SMB are added. Robustness is ensured by extending the MCTF framework with empirical infrastructure tests for each backend.