Add database persistence for Offerings

[juanrasantana]

At this moment, there is no data persistence. Thus, service reboot will erase created Offerings. Adding persistence is necessary, but it should be using a similar approach as the EDC Connector uses (a simple candidate is PostgreSQL) for future integration.

[franckLG]

No clue about what EDC connector use but as I understand the assets descriptions will be in NGSI-LD, the offerings could be persisted as NGSI-LD as well (then you could build some subscription based workflows upon Offerings evolution)

[xamcost]

Bouncing on @franckLG 's comment, and considering that in D5.3 we mentioned that tools like MageAI will be integrated with the NGSI-LD broker to store their assets description, I would suggest harmonising, i.e. having either MageAI use the offering manager, or the offering manager using the NGSI-LD broker to store its offerings, otherwise we may increase unnecessarily the complexity and technical debt of Sedimark by having to handle two storage for the same objects.