feat: re-add updates to serde of GeneratedAnswer and ExtractedAnswer

[sjrl]

Related Issues

• fixes https://github.com/deepset-ai/haystack-private/issues/327
• related to fix: updating generated answer and extracted answers serialisation #11805 and docs: fix replacement locations in MIGRATION.md Generators-removed section #11948

Proposed Changes:

Re-adds changes made in #11805 and also updates the from_dict methods of the two dataclasses to support the old serialization format.

How did you test it?

New tests

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have 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: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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Project	Deployment	Actions	Updated (UTC)
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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/dataclasses
answer.py
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[julian-risch]

Looks good to me. It's good that the release note points out that any code that calls .to_dict() and then reads result["init_parameters"]["data"] will break because it's now a flat dict with ({"data": ..., "query": ..., ...}). To that extent this PR still is a breaking change.

[sjrl]

To that extent this PR still is a breaking change.

Yes that is correct