Review how secrets are handled

[enolfc]

Short Description of the issue

We have secrets in several places in this code and they are treated in different ways (via a file, via env variables, in GitHub Actions secrets, ...), we should review how these are managed and move to as simple and secure way as possible.

[gwarf]

For the record: recently Bitwarden started to provide a service meant to manage this kind of secrets: https://bitwarden.com/products/secrets-manager/.

[brucellino]

Seems like secrets.egi.eu would be the obvious choice?