[Automated ingestion] Map S3 buckets to CVMFS repositories

[bedroge]

This allows you to specify a dictionary of buckets in the configuration, and map them to CVMFS repositories, e.g.:

staging_buckets = {
    "software.eessi.io-2023.06": "software.eessi.io",
    "dev.eessi.io-2024.09": "dev.eessi.io",
    "riscv.eessi.io-20240402": "riscv.eessi.io" }

The ingestion script has been adjusted as well: you now have to specify to which repo a tarball has to be ingested. The tests for this script have been adjusted accordingly:

$ ./test-ingest-tarball.sh 
Ran 31 tests, 31 succeeded, 0 failed.

Finally, the automated ingestion now also prints the target CVMFS repo in the PR title+description, and in the Slack message.