fix: tell type checkers that the config options are strings

[tonyandrewmeyer]

Description

ops<=2.12 wrongly has self.config[x] typed as str, when actually it could be an int, float, bool, or str, depending on the config type. We're fixing this in ops:#1183, but that will break static checking that currently assumes that the config is a str (because ops doesn't validate the schema, so all options will be bool|int|float|str).

This PR adds a typing.cast call where the config values are loaded and used where the type should be str.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation N/A
• I have added tests that validate the behaviour of the software N/A
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules N/A
• I have bumped the version of the library N/A