Is your feature request related to a problem? Please describe.
Currently, the only way to load prompts during genkit.Init is by passing a path to WithPromptDir, which means one has to copy their prompt directory together with the final binary to use it. allowing filesystems created via embed means one can ship a self contained binary without the hassle of copying over the prompts directory.

Describe the solution you'd like
Add support for embedded filesystems created via embed.

Describe alternatives you've considered
A workaround is to embed the prompt directory then during runtime unpack it before initializing genkit. this may lead to permission issues whoever.

Additional context
None