refactor: Collect all status in one place

[dashmage]

• Set all statuses from the collect_unit_status event handler.
• Change name from exporter_enabled to relation_present
• Refactor code into render_exporter_config
• Change return format for validate_exporter_configs to be simpler.

[dashmage]

A lot of unit tests break with this change. Once the overall logic is finalized, I'll go ahead and make the required unit test modifications.

[dashmage]

I'm thinking whether I should make changes to resolve https://github.com/canonical/hardware-observer-operator/issues/201 as well since I'm anyway changing the stored state variables with this PR.

[Pjack]

Close this PR, since it was pending for a while.