nixos: overhaul datadog module

This overhauls the Datadog module a bit to be much more useful. In
particular, it adds support for nginx and postgresql monitoring
integrations to dd-agent. These have to exist in separate files under
/etc/dd-agent, so the module just exposes then as separate options. In
the future, more integrations could be added this way.

In the process of doing this, I also had to rename the dd-agent user to
datadog. Note the UIDs did not change, so this is strictly backwards
compatible. The reason for this is to make it easier to create a
'datadog' postgres user with access to pg_stats, as 'dd-agent' typically
isn't a valid username. This allows the out of the box configurations to
be used.

Signed-off-by: Austin Seipp <aseipp@pobox.com>