Expose multiId field in subscriptions

[achim-k]

Changelog

Expose multiId field in subscriptions

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Description

Adds the multiId field from the Subscription Message to the value type of the map returned by the subscriptions getter.

multi_id: the same message format can have multiple instances, for example if the system has two sensors of the same type. The default and first instance must be 0.

With out this field, the consumer is not able to tell from the subscriptions map which message id corresponds to which message format instance.