feat(bedrock): add prompt caching for supported models

[mfenderov]

Prompt caching is automatically enabled for models that support it (detected via models.dev) to reduce latency and costs. System prompts, tool definitions, and recent messages are cached with a 5-minute TTL.

To disable:

provider_opts:
  disable_prompt_caching: true

P.S. Benchmarked with examples/pr-reviewer-bedrock.yaml: 92% cache read vs 8% cache write.

Assisted-By: cagent

[krissetto]

This generally looks good to me, thanks for the contribution! ❤️

@dgageot since you've been deep diving on anthropic caching bits lately, you might be interested in taking a look here and maybe comparing bedrock to some of the tests you've been doing with anthropic's api