feat: add Jinja2 ChatMessage extension

[anakin87]

Related Issues

• part of ChatPromptBuilder adaptation for multimodality haystack#9260: I'm not yet modifying the ChatPromptBuilder but introducing the foundations to make it work

Proposed Changes:

• introduce a Jinja2 ChatMessage extension
  1. The {% message %} tag is used to define a chat message.
  2. The message can contain text and other structured content parts.
  3. To include a structured content part in the message, the | for_template filter is used.
     The filter serializes the content part into a JSON string and wraps it in a <haystack_content_part> tag.
  4. The _build_chat_message_json method of the extension parses the message content parts,
     converts them into a ChatMessage object and serializes it to a JSON string.
  5. The obtained JSON string will be usable in the ChatPromptBuilder component, where templates are rendered to actual ChatMessage objects.

How did you test it?

CI; several tests for the extension

Notes for the reviewer

• Don't get scared by the size of this PR (there are many tests)
• (I'll keep working on the ChatPromptBuilder using this basis)

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

[coveralls]

Pull Request Test Coverage Report for Build 14929353715

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.1%) to 70.654%

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Files with Coverage Reduction	New Missed Lines	%
dataclasses/chat_message.py	2	98.31%

Totals
Change from base Build 14918586053:	2.1%
Covered Lines:	1134
Relevant Lines:	1605

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💛 - Coveralls

[sjrl]

Also more of just a question. Why the name for_template? I'm not yet quite understanding what it does based on just the name.

[anakin87]

Good point. I would like a short but descriptive name.
Other ideas: content_part/prepare_content_part/mark_content_part/format_content_part; something related to "including".

Do you have any other ideas?
(We can also involve some other team members if necessary)

[sjrl]

Yeah I'm not entirely sure either. I'd be happy to have other team members give their input

[sjrl]

@anakin87 Overall looks really good! I have some some comments mostly surrounding tests and error handling but the templating language itself looks very solid!

[sjrl]

Everything looks good! I don't think the for_template naming needs to block this. Will leave it up to you