feat: Bedrock - support prompt caching

[anakin87]

Related Issues

• part of Add prompt caching for AmazonBedrockGenerator and AmazonBedrockChatGenerator #2776 (to complete this, I also paln to add a small section about Prompt caching in docs)

Proposed Changes:

• allow setting cache points in messages
  ChatMessage.from_user("long message..."), meta={"cachePoint": {"type": "default"}}
  • aligns with Bedrock docs
  • The documentation is ambiguous about multiple cache points per message. Implementing this would be hard/hacky within the current ChatMessage abstraction, so this PR only supports one cache point per message.
  • we also discussed in Add prompt caching for AmazonBedrockGenerator and AmazonBedrockChatGenerator #2776 (comment) about adding a global parameter to the Chat Generator to cache all user/system messages. However, Bedrock allows a maximum of 4 cache points per request, so per-message cache points are more transparent and allow finer control.
• add a tools_cachepoint_config init parameter to the ChatGenerator to enable caching for tools
  • Bedrock may support more granular caching for tools, but it is hard to implement while respecting Haystack API, so I adopted this simpler solution.

How did you test it?

CI, new unit and integration tests.

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:.