feat: add cached token metrics support for Amazon Bedrock

[poshinchen]

Description

Add cached token metrics support for Amazon Bedrock to track cache read/write input tokens.

Taking over the pr with tests and some refinements

Related Issues

#529

Documentation PR

N/A

Type of Change

Update

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Unshure]

This import does not look correct, can you fix it?

• https://github.com/strands-agents/sdk-python/blob/main/src/strands/telemetry/metrics.py#L14

• That import comes from here: https://github.com/strands-agents/sdk-python/blob/main/src/strands/types/streaming.py#L13