Add PromptBuilder

[silvanocerza]

Related Issues

• Closes #5622

Proposed Changes:

Add minimal PromptBuilder implementation to render string templates to prompt strings so they can be passed to Generator Components.

Usage:

from haystack.preview.components.generators.prompt_builder import PromptBuilder

template = "Translate the following context to {{ target_language }}. Context: {{ snippet }}; Translation:"
builder = PromptBuilder(template=template)
res = builder.run(target_language="spanish", snippet="I can't speak spanish.")

assert res["prompt"] == "Translate the following context to spanish. Context: I can't speak spanish.; Translation:"

How did you test it?

Added unit tests.

Notes for the reviewer

I didn't add support for PromptHub as we still have no prompt templates using the new format.
Also it can be easily added in the future and I wanted to keep the PR small.

I already added Jinja2 as a dependency in haystack-ai package.

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[ZanSara]

Nice! A couple of nitpicks, then I'll approve

[ZanSara]

🚢