[Housekeeping] Provide realm auto deletion functionality

[nanjarapalli]

Problem

• realm can be deleted either from housekeeping restapi (only when no devices are connected)
• using astartectl

When evaluating the astarte with different realms it becomes difficult to maintain all the realms and cleaning the unused realms. Hence requesting to extend housekeeping ream creation api to include TTL flag/paramter to auto delete the realm when set during realm creation

[Annopaolo]

Realm deletion implies data loss, so an automatic deletion based on TTL could open a number of problems (e.g. what about online devices whose realm has been auto-deleted?). We should carefully consider if we want to provide such a feature.