feat: add required flag for prompt builder inputs

[LastRemote]

Related Issues

• changes PromptBuilder behavior according to All input parameters in PromptBuilder are optional #7441

Proposed Changes:

PromptBuilder now takes a new optional parameter required_variables upon initialization to specify a list of jinja template variables that must be provided at all times. During pipeline or component runs, a properValueError will be thrown if some of the required variables are missing.

How did you test it?

Added a new unit test.
Tested by following the 2.0 RAG pipeline recipe from https://docs.haystack.deepset.ai/docs/creating-pipelines

Notes for the reviewer

For the sake of completeness, I also added a validation check in PromptBuilder.run().

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

[CLAassistant]

All committers have signed the CLA.

[LastRemote]

Previous PR: #7442 . I devided to make a new one since this should not be considered a fix anymore.

[coveralls]

Pull Request Test Coverage Report for Build 8715948805

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 90.073%

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Totals
Change from base Build 8664841004:	0.01%
Covered Lines:	6261
Relevant Lines:	6951

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💛 - Coveralls

[silvanocerza]

Nice, thanks! 🚀