PCI database embedded fallback

[BenTheElder]

See: #194 (comment) and this thread: #194 (comment)

I think we should probably read a default path to see if there is a DB mounted there from the host or the image etc as deployed by the user, and if not fallback to an embedded copy.

For prior art: https://pkg.go.dev/time/tzdata

We can also continue to support a flag or env for overriding the path.

We should probably assume that if a non-embedded copy is available, it is preferable.