Implement I/O from/to S3

[holtgrewe]

Is your feature request related to a problem? Please describe.
VarFish shall work on S3 storage internally. For this, the commands should be able to read from and write to S3 storage.

Describe the solution you'd like

• Interpret the environment variables from below and enable S3 mode in this case.
• Interpret absolute paths to always come from the local file system.

Describe alternatives you've considered
N/A

Additional context
Environment variables (cf. aws cli docs)

• AWS_ACCESS_KEY_ID -- must be set, eanbles s3 mode
• AWS_SECRET_ACCESS_KEY -- must be set
• AWS_ENDPOINT_URL -- must be set
• AWS_REGION=us-east-1

[holtgrewe]

OK, so direct upload to S3 is not possible as you can only easily stream upload from a file.