feat: add schema validation to promptfooconfig.yaml

[mldangelo]

This update introduces schema validation to promptfooconfig.yaml (in eval only). Currently, it issues a warning if the schema validation fails, using a standard Zod warning. I tested this on all of our example promptfooconfig.yaml files with a script like the following:

#!/bin/bash

# Find all promptfooconfig.yaml files
config_files=$(git ls-files | grep promptfooconfig.yaml)

# Loop through each config file and run promptfoo eval
for config in $config_files
do
  echo "Running evaluation for $config"
  promptfoo eval -c "$config"
done

echo "All evaluations complete"

I made a slight modification to process.exit() in eval.ts.

In the future, we should:

• Add description fields to the Zod schema
• Include more transform steps to simplify downstream dynamics
• Generate a JSON schema that we can integrate into text editors to provide users with instant validation

CC @cmrfrd