Fixing bugs in prompt rendering

[maxl0rd]

This fixes the bug reported in https://github.com/firebase/genkit/issues/422 where renderPrompt does not preserve the output schema that the prompt function returns.

This also fixes the bug reported in https://github.com/firebase/genkit/issues/757 where Dotprompt.render() only accepts default options. With this change, the render and generate methods will accept any options, unless a model is also passed in the method, in which case it requires the custom options defined by that model.

Checklist (if applicable):

• [ ] Tested (manually, unit tested, etc.)
• [ ] Changelog updated
• [ ] Docs updated