Feature request: store source of passively discovered subdomains in database

[kz0ltan]

It was really useful to know the source of discovered subdomains (with this I mean how the asset was discovered) in the previous version 3, I think it would be useful for v4 as well.

Currently the schema for assets it this:
id|created_at|type|content|last_seen

By adding a source field, bad sources could be investigated.
Most probably I could do the change myself, but I see many pull requests in the queue and I'm not sure if this change would be even considered.
Thank you!