Implement the concept of Credentials/Secrets

[vpereira]

Use case:

As OBS user, I want to be able, among other things, to pass credentials to a "_service" to download files in a secure way.

Another use case would be to be able to inject this credential in a container in a safe way, while building it.

Examples:

• https://docs.gitlab.com/ee/ci/secrets/
• https://docs.github.com/en/actions/security-guides/encrypted-secrets