Add support for signed tasks.

[hooverdc]

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[hooverdc]

Thanks, I'll get working on those.

[hooverdc]

@RealOrangeOne Added tests and made updates based on your suggestions.

[RealOrangeOne]

CI is still failing. However, on reflection, I'm not sure the added complexity is worthwhile.

Because task functions must be explicitly allowed, there's no ability to gain remote-code execution inside Django (at least not in ways which can be controlled by django-tasks). Whilst it's possible to potentially modify data, if an attacker already has access to the database they can do that anyway, not to mention that they probably have access to other parts of your infrastructure too.