Add provider and requirer test charms for testing

### Context

It would be ideal to have a provider and requirer charms that allows libraries in this repository to be tested, as well as users of the libraries. Ideally we'd have both ends of the relations that the libraries would handle.

### What needs to get done

Create "dummy" charms that can be used for testing purposes.

### Definition of Done

Have charms that emulate the provider and requirer sides of the relations in this libraries and are publicly available for others to use.

Start by reviewing the repository's existing libraries and the relations they handle to identify the provider and requirer sides that need dummy charms. Create publicly usable test charms for those relation ends, then verify that library tests and consumer tests can exercise both sides as described in the definition of done.

Written by the indexing model from the issue text.