Implement auth proxy relation and access rules rendering logic

[natalian98]

Summary

This PR:

• adds auth-proxy relation interface
• adds a peer relation to store access rules locations for each relation
• adds access rules rendering and removing logic
• modifies config template

[natalian98]

I have only one question, why do we need to store the file locations in the peer relation? Couldn't we somehow construct the paths from the relation ID? I suppose that once we move to configMaps this logic will change so we don't need to change it.

I put the relation id along with its files locations mostly to cater for relation-departed events for both auth-proxy and the forward-auth interface we'll implement next. I think it's better to store this data in a peer relation in case the relation id is unaccessible but we still need it to fire another custom event (update forward-auth), or if a charm got only deny rules created, for example.