S3 support

[d3banjan]

• closed discussion Add S3 compatible artifact service google/adk-python#654
• PR looks stalled. feat: Add S3 support for artifacts google/adk-python#556

[d3banjan]

Notes

• missing boto3 in requirements
• pip install -e ".[test]" for test extras
• tests pass after installing boto3

Ideas

• service targets should be extras too?