feat(object_storage): add multi-provider client with auto-detection

[jplbrun]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

Adds a new sap_cloud_sdk.object_storage package that provides a single, provider-agnostic client for object storage across Amazon S3/MinIO, Azure Blob Storage, and Google Cloud Storage.

Users call create_client(instance=...) and get back a client satisfying the ObjectStoreClient protocol. The concrete provider is auto-detected from the service binding, so consuming code stays identical regardless of which cloud backs the instance.

Highlights:

• Unified interface — one ObjectStoreClient protocol (put_object, put_object_from_bytes, put_object_from_file, get_object, delete_object, list_objects, head_object, object_exists) implemented by all three provider backends.
• Automatic provider detection — create_client() infers the provider from the binding keys present in the secret mount / env vars. Detection requires a complete provider signature and errors clearly when keys match zero or multiple providers.
• Explicit config escape hatch — pass S3Config, AzureConfig, or GcsConfig directly to create_client(config=...) to bypass auto-detection.
• Config resolution follows the SDK's standard precedence (flat mount → legacy mount → env vars), consistent with the rest of the SDK.
• Legacy objectstore module is now deprecated — its operations are marked deprecated, constructing the legacy client emits a DeprecationWarning pointing to the new package, and its user guide / README link are updated accordingly. The old module still works for a smooth migration.
• New runtime dependencies for the Azure and GCS backends (azure-storage-blob, google-cloud-storage), so all three providers work out of the box.

This ports and generalizes the work from #291 into a dedicated multi-provider package.

Related Issue

N/A

Type of Change

Please check the relevant option:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. uv sync --all-extras --group dev
2. Run the unit suite: uv run pytest tests/object_storage/unit -q — covers detection, factory dispatch, per-provider clients, config resolution, validation, the public API surface, and the legacy deprecation warning.
3. Run the integration suite against real bindings per docs/INTEGRATION_TESTS.md: uv run pytest tests/object_storage/integration -v.
4. Expected result: all unit tests pass; create_client() returns the correct backend for each provider binding; the legacy objectstore client emits a DeprecationWarning.

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Breaking Changes

None. The new object_storage package is purely additive. The existing objectstore module is deprecated but continues to function;

Additional Notes

None