Support go's embed FS

### What problem are you facing?
"FileSystem" Templates loaded from configmaps is a wonky user experience which makes them not as extendable as possible.

### How could this Function help solve your problem?

If you support go's embed it will allow templates to actually be loaded from filesystem and embedded into the function as a virtual file system which is available at runtime.

https://pkg.go.dev/embed

Start by reviewing the current template-loading path for configmaps and the Go embed package documentation. Determine how an embedded virtual filesystem should be exposed at runtime, then verify that templates can be loaded from it while preserving the existing configuration behavior.

Written by the indexing model from the issue text.