feat: Load Jinja template from external file location

[SoloSynth1]

This PR:

• adds TemplateLoader.load_from_external() for loading external template with optional validation logic
• adds optional argument in ResponseParser to allow use of non-default template when exporting EvaluationResponse.
• adds tests for TemplateLoader

[JohnShiuMK]

Thanks @SoloSynth1. But I think the export_template.filename is empty.

Do you think it's better to use Environment(loader=FileSystemLoader(...)) in the load_from_external?

[JohnShiuMK]

nice!