Story, design and implementation for secrets management

### Is your feature request related to a problem? Please describe.

Polaris does requires secrets like storage credentials and access credentials for remote systems.

Those secrets can currently only be configured statically via the configuration or persisted, latter is rather static as well. Also, secrets are currently persisted in various very different ways / property keys.

It would be much better to have a consistent way to access secrets and (let them) manage those in a secure way. At best, secrets would not even be stored in Polaris, but managed by trusted secrets managers.

All secrets should be considered "ephemeral" - meaning: secrets can be rotated at any time without even noticing Polaris. Already assuming in the design that secrets can be rotated at any without being noticed makes it easier for users and system integrators.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

No files, tests, or entry points are named. Start by mapping how Polaris currently configures and persists storage and remote-system credentials, then define the consistent ephemeral secret-management design and implementation scope; done requires an agreed design and the resulting implementation.

Written by the indexing model from the issue text.