Feature Request: Add cachePoint support for langchainjs aws lib

[tnlamda]

Checked

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Feature request

According to AWS Documentation, cache checkpoints are now supported by Claude 3.5 Haiku/Sonnet v2 as well as Amazon's Nova models. Cache checkpoints can be added to the system field for all of these models. However, langchain only supports strings for system messages, see here.

Motivation

My team has been given access to the new prompt caching feature, but we're unable to use it due to langchain's lack of support in the js library. A similar pull request has been made for adding cache checkpoints to the python lib.

Proposal (If applicable)

The system message validation needs to be updated to support the new cache checkpoint feature, then properly pass it along to Converse- I suspect it would already get passed along fine.