This adds the command itself, and the supporting method in the store layer. For ListLibCommand: - it can run standalone, or with a given charm name - when standalone, it will try to access metadata.yaml, and if there it will list all libraries for that charm - if a charm name is indicated, will just list all libraries for that charm In the store layer: - a new multi-functional `get_libraries_tips` is added - it's bulk, and supports all the flexibility in the store for each query: - just the library id (will get the tip for that library) - just the charm name (will get the tip for all libraries of that charm) - charm and library names (alternative identification of a specific library) - for any of those cases, `api` is supported (it will give the tip for that API version) - note that this is current more flexible than needed for ListLibCommand itself, but more of this will be used in the FetchLibCommand (in the next branch)