Add support for subscriptions to all instances of an interface for subscribing to variables

[james-ctc]

Currently you subscribe to a specific module instance for a particular interface.
e.g.

evse_manager->subscribe_session_event([](types::evse_manager::SessionEvent event) {...});

There can be more than one EVSE manager (up to 128) and each one would require:

• a link in the charger YAML configuration file to the module wanting to subscribe
• a separate subscription for each EVSE manager

The improvement is to allow a "wildcard" subscription so that a single subscribe would provide all session events regardless of which EVSE manager raised it.

e.g. a subscribe which provides the session event (variable) along with the EVSE manager ID that published it.

mqtt->subscribe_all_session_events([](const std::string &evse_manager_id, types::evse_manager::SessionEvent event) {...});

It might be possible to further generalise the approach so that other interface variables can be subscribed to without having to bind to a specific instance.

[james-ctc]

idea came from PR #215