TemplateRegistry support add custom jinja2 template loader

[rongfengliang]

Currently ,TemplateRegistry only soppurt j2.PackageLoader and j2.FileSystemLoader, maybe we can pass custom jinaj2 loader for better storage and load prompt template contents (Templates maybe storage in s3,gcs )

[groeney]

Yes definitely and thank you for filing this! This was one of those things that we wanted to wait and see if there was any demand for before implementing it. Will take a pass at it soon—currently ooo :/