Handle saving/loading the set of tags

[Yoric]

For the moment, tags live only in memory. We should be able to save them to disk. Basically, each id may be associated to a list of strings.

This means adding to manager::AdapterManager methods

pub fn load(&self, path: String) -> Result<(), Error>;
pub fn save(&self, path: String) -> Result<(), Error>;

Then, when we perform the internal calls to add_service, add_getter or add_setter, we should check in the in-memory list to find out if the service/getter/setter has tags waiting for it.

I'm not sure about the error type.

In a followup issue, we'll think about saving tags as they are added/removed, or associating tags to