Add support for pebble log forwarding

[lucabello]

Issue

Closes #333.

Solution

The solution has changed so many times, you'll probably have a better understanding by reading the documentation additions in the PR.

[lucabello]

TODO: make sure that the class is observing logging relation (or relation_name) changed and if there's a loki endpoint it should enable log forwarding automatically.

A client charm that instantiate this and relates to Loki directly, should be able to only call the constructor and have things work out of the box.

[IbraAoad]

Unit tests are still missing validations of the actual pebble layer due to this canonical/operator#1111

[PietroPasotti]

I see some issues with the API and a bunch of improvements that can be made on the documentation side. Take a look and let me know!

[PietroPasotti]

Still some unclarity left in the updated API.

[Abuelodelanada]

Shouldn't we have itest that verifies this feature?

For instance would be nice to have an itest that checks if you have a charm with 2 containers everything works

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