Support labeling artifacts while uploading them

[joao-paulo-parity]

Problem: We'd like to be able to label artifacts while uploading them. Those labels can be queried later to filter e.g. all files belonging to a given architecture, or all binary files, all metadata files, etc.

Solution: support the --label option.

For S3 this translates to using --metadata (https://docs.aws.amazon.com/cli/latest/reference/s3/cp.html)