feat: Add AnswerJoiner new component

[vblagoje]

Why:

Adds an AnswerJoiner to merge multiple lists of answers using various join modes, addressing the need for simplified answer aggregation.

What:

1. Added an AnswerJoiner class in haystack/components/joiners/answer_joiner.py for merging lists of answers.
2. Modified haystack/components/joiners/__init__.py to include AnswerJoiner in the module exports.
3. Added unit tests in test/components/joiners/test_answer_joiner.py to validate the functionality of AnswerJoiner.

How can it be used:

• Similar to DocumentJoiner

How did you test it:

• Conducted unit tests including initialization tests, to/from dictionary serialization, empty list handling, and checking for correct answer merging.
• Verified custom join functions and error handling for unsupported join modes using pytest.

Notes for the reviewer:

• Review the JoinMode enumeration in answer_joiner.py to ensure future proofing for additional join modes.
• Check the logic within _concatenate method for performance implications with large lists.
• Validate the custom join function handling to ensure robustness against edge cases.

[coveralls]

Pull Request Test Coverage Report for Build 10196642167

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 90.091%

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Totals
Change from base Build 10177216364:	0.04%
Covered Lines:	6864
Relevant Lines:	7619

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💛 - Coveralls

[vblagoje]

Looking for some early feedback before I wrap up @sjrl
Note the option to add custom_join_function as I found concatenate only to be limiting for all different types of Answers and potential use case scenarios. With custom_join_function we can grow this component and then standardize emerging join functions by adding them next to concatenate. This way we get the best of both worlds.
LMK

[sjrl]

Hey @vblagoje as long as the custom join function is specifiable in a yaml file then I'm all aboard making it easy to extend in the future.

[sjrl]

So @vblagoje if you could add a test for to_dict, from_dict and also loading the component from a yaml file then the custom function all sounds good to me :)

[vblagoje]

Note to @vblagoje @dfokina - we need to add this component to documentation, not sure if worthy of social but docs definitely

[vblagoje]

@sjrl while I'm on PTO please feel free to move this one forward, anyone else welcome as well 🙏

[sjrl]

Hey @Amnah199 I'd really appreciate your review on this, since I've made some changes so it feels odd for me to approve my own work 😅

[sjrl]

Hey @dfokina could you look over the doc strings to check if this okay with the recent effort of rehauling doc strings for components?

[Amnah199]

Looks good!

[dfokina]

Thank you for tagging me @sjrl , going over it now!

[dfokina]

Docstrings updated