Add first seen field to host and beacon ent.

**Is your feature request related to a problem? Please describe.**
For red team deconfliction we'd like to know when we first accessed a host and when a beacon restarted.

**Describe the solution you'd like**
To get this we should add a "first_seen_at" field to the beacons and hosts.

**Describe alternatives you've considered**
Na 

**Additional context**
Na