feat: allow mellea programs to be added as tools

[jakelorocco]

Mellea programs are nominally functions that accept a backend, context, etc... as their parameters. We want to enable adding these mellea programs as tools in the context of other calls. We will need special logic to handle the threading of the backend and context to these tools (as well as formatting them to be invoked correctly by the model (ie the model shouldn't have to specify the backend / context to call)). We will also need to consider if model_options, any other parameters should be passed along as well.

[jakelorocco]

This will allow us to more easily compose mellea programs into complex agentic patterns.