feat(go): support using a custom filesystem to load prompts from #3746

[naqerl]

Description here... Help the reviewer by:

• Draft implementation for #3746
• Sample prompts-dir was updated with the new API
• Docs will be updated after getting approve on this feature

Checklist (if applicable):

• [x] PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• [x] Tested (manually, unit tested, etc.)
• [ ] Docs updated (updated docs or a docs bug required)

[hugoaguirre]

Hi @naqerl, thanks for your contribution. We will discuss this PR internally and come back to you!

[josearomeroj]

Any update on this?