Support authorized_keys lookup via DB

In gitlab-com/operations#99 (closed), we have a method of looking up a user's keys from the database instead of the authorized_keys file.

We have a number of large-scale customers who could really benefit from this. Can we make this an omnibus-supported EE option? Do we have documentation for this, @pcarranza?

ZD: https://gitlab.zendesk.com/agent/tickets/71687

/cc: @MrChrisW, @dblessing