dd-agent: Simplify inclusion of additional Datadog core integrations

Refactors the process used to build the Datadog core integrations to
be more easily extensible with integrations other than the ones built
and installed by default.

Documentation has been added in relevant parts of the module to
describe how the process works.

As a high-level overview:

The `datadog-integrations-core` attribute in the top-level package set
now accepts an extra parameter.

This parameter is an attribute set where each key is the name of a
Datadog integration as it appears in Datadog's integrations-core
repository[1], and the value is a function that receives the Python
package set and returns the required dependencies of this integration.

For example:

    datadog-integrations-core {
      ntp = (ps: [ ps.ntplib ]);
    };

This would build the default integrations and, additionally, the `ntp`
integration.

To support passing the modified Python environment to the
datadog-agent itself, the `python` key has been moved inside of the
derivation which means that it will be made overridable.

This relates to NixOS/nixpkgs#40399.

[1]: https://github.com/DataDog/integrations-core