Introduce dspy.Template

[KCaverly]

The original implementation of TemplateBackend relied upon the signature_to_template function. This would take in a dspy.Signature and return a dsp.Template. However, we would pass dspy.Example into the dsp.Template and the extract function would return dsp.Example a different type. Leaving us with two different types of Example within a single prediction. To combat this, a new Template primitive was introduced into dspy to mirror and replace the original dsp.Template.

I do not expect this current PR to achieve feature parity with the original class, but initial tests have been introduced to mock and mirror the current prompt structure used in dsp.

Let me know what you think @CyrusOfEden, the last piece I believe to start using the TemplateBackend is to fully explore the do_generate function dealing with incomplete or inaccurate completions.

[CyrusNuevoDia]

YES man this is 🔥