move gcs and s3 deps to extras

[hirosassa]

Summary

• Move S3 (boto3) and GCS (google-auth, google-api-python-client) dependencies to optional extras, installable via pip install gokart[s3], pip install gokart[gcs], or pip install gokart[all]
• Remove unused dependency uritemplate
• Lazy-load S3/GCS-related imports, raising a clear ImportError with installation instructions when extras are missing

Motivation

pip install gokart previously bundled cloud storage libraries even when unused, increasing package size unnecessarily.

• S3 extras (boto3 + transitive deps)
• GCS extras (google-api-python-client + transitive deps)

By making these optional, users who don't need S3/GCS can reduce dependencies by up to ~109 MB.

Dependency size impact

Compared to gokart[all] (~110 MB of extras dependencies):

Install command	Extras included	Savings vs [all]
pip install gokart	none	-110 MB
pip install gokart[s3]	S3 only (~24 MB)	-86 MB
pip install gokart[gcs]	GCS only (~86 MB)	-24 MB
pip install gokart[all]	both (baseline)	—

Breaking Changes

S3/GCS functionality is no longer available with a bare pip install gokart. Users must opt in:

pip install gokart[s3]   # S3 only
pip install gokart[gcs]  # GCS only
pip install gokart[all]  # both

Changes

• pyproject.toml: Move boto3, google-auth, google-api-python-client from dependencies to optional-dependencies.
• s3_config.py, gcs_config.py, object_storage.py, gcs_obj_metadata_client.py, s3_zip_client.py, gcs_zip_client.py: Convert top-level imports to lazy imports with helpful error messages.
• test/test_extras.py: Add tests verifying behavior with and without extras installed.

[hiro-o918]

LGTM!

[kitagry]

Sorry for the late review 🙏

[kitagry]

Could you add something like the following? I think it would be better to add type annotations.

if TYPE_CHECKING:
    import luigi.contrib.s3