[23571] Greedy discovery mode

[emiliocuestaf]

This PR adds support for Proxy Entities. Proxy entities are those that are discovered when a PROXY message with an unknown GUID arrives to the backend. That behavior can be achieved through a router or other forwarding tools.

• When the backend receives a PROXY message related to an unknown entity, it populates its discovery information and request an update of the entity database.
• Added a new DiscoverySource enum to represent how an entity was discovered (unknown, discovery, proxy), and updated entity creation functions to accept and store this information.
• Updated JSON tags and serialization logic to include the discovery source for all relevant entities; this ensures the discovery source is present in API responses and database dumps.
• If proxy entities are later discovered via the standard procedure, their discovery_source is updated.