feat: add author-pluggable extension point for provider-specific message fields

[AngeloDanducci]

Pull Request

Issue

Fixes #1565

Description

Allows author-pluggable fields via provider_fields.

Given the exploration around the original issue, I think this gives us the most extensibility for the least amount of headache.

I chose provider-keyed passthrough instead of widening format_for_llm as this leaks wire shape into component code and doesn't compose well with images/docs (same issue with a global serializer registry). Open to discussion though if anyone can see reasons to alter the design - ie if we think authoring custom serialization logic itself is of greater benefit/immediate need.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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