add a lib for juju topology

[simskij]

Issue

This PR adds a common library for JujuTopology. It strips away all of the complexity we've added in other charms in favor of a simple and coherent API.

Solution

• Get rid of the abstract base class.
• Get rid of the mix between properties and methods.
• Add the ability to exclude items from the dict
• Add the ability to remap keys in the dict

Context

N/A

Testing Instructions

• Try the library with one of our existing charms, replacing the current implementation with the one in the library.
• Run all tests, both unit and integration.
• Perform manual sanity checks

Release Notes

Adds a reusable library for creating, generating and formatting Juju topology.

[jnsgruk]

This could also do with a slight adjustment to the top-level README to add a reference to this lib :)

[simskij]

This could also do with a slight adjustment to the top-level README to add a reference to this lib :)

Good catch! Thanks!