Exp s3 with env interfaces

[javierdelapuente]

Applicable spec:

Overview

Leaving here just for reference.

Instead the implementation will be different, as different "application managers" like WsgiApp will generate different env variables, instead of being sent through the charmstate.

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• [ ] The charm style guide was applied
• [ ] The contributing guide was applied
• [ ] The changes are compliant with ISD054 - Managing Charm Complexity
• [ ] The documentation is generated using src-docs
• [ ] The documentation for charmhub is updated
• [ ] The PR is tagged with appropriate label (urgent, trivial, complex)