nixos/datadog-agent: Refactor to allow arbitrary check configs

Refactors the datadog-agent (i.e. V6) module to let users configure
arbitrary checks, not just a limited set, without having to resort to
linking the files manually and updating the systemd unit.

Checks are now configured via a `services.datadog-agent.checks` option
which takes an attribute set in which the keys refer directly to
Datadog check names, and the values are attribute sets representing
Datadog's configuration structure.

With this mechanism users can configure arbitrary integrations, for
example for the `ntp`-check, simply by saying:

    services.datadog-agent.checks.ntp = {
      init_config = null;
      # ... other check configuration options as per Datadog
      # documentation
    };

The previous check-specific configuration options for non-default
checks have been removed. Disk & network check configuration options
have been kept rather than making them a `default`-value of the
`checks`-option because they will be overridden by user-configurations
in that case.

Relates to NixOS/nixpkgs#40399.