add public method to LogForwarder to retrieve the full endpoint urls

[PietroPasotti]

we only have a private: _retrieve_endpoints_from_relation method.

To facilitate integration with e.g. charm logging utils, we should be able to access the endpoints from the parent object (the charm).

Originally posted by @Abuelodelanada in #392 (comment)

[lucabello]

That function was intentionally made private because we didn't want charms to use that. We can go back on that decision, but I'd rather have an ADR on that to make sure the team's on the same page :)