⚙️ FEATURE-#115: Add StorageGCS provider for Google Cloud Storage persistence

[FernandoCelmer]

Description

• pyproject.toml: Added google-cloud-storage as optional dependency (pip install dotflow[gcp])
• dotflow/providers/storage_gcs.py: New StorageGCS provider implementing the Storage ABC with GCS backend, bucket.reload() validation on init
• dotflow/providers/init.py: Lazy import for StorageGCS via __getattr__
• tests/providers/test_storage_gcs.py: 8 unit tests using unittest.mock
• docs_src/storage/storage_gcs.py: Usage example
• docs/nav/tutorial/storage-gcs.md: Tutorial page with authentication guide
• docs/nav/reference/storage-gcs.md: API reference page
• mkdocs.yml: Updated nav with StorageGCS pages

Motivation and Context

Enables serverless and cloud-native workflows on GCP by persisting task context to Google Cloud Storage. Same pattern as StorageS3 (#104) but for Google Cloud. Prerequisite for Cloud Run / Cloud Functions execution.

Closes #115

Types of changes

• New feature (change which adds functionality)
• Documentation

Checklist

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the CHANGELOG
• I have updated the documentation accordingly