(bedrock): Missing Bedrock Guardrail association for CfnBedrockAgent construct

### Describe the feature

Hi AWS folks,

I'm not sure if this feature request should be submitted here given this seems to be lacking from the [CloudFormation API](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-bedrock-agent.html) as well.

Bedrock Agents support associating Bedrock Guardrails via a `guardrailConfiguration` parameter as [described in the API docs](https://docs.aws.amazon.com/bedrock/latest/APIReference/API_agent_CreateAgent.html#bedrock-agent_CreateAgent-request-guardrailConfiguration). This doesn't seem to be a supported property in `CfnAgent` yet.

### Use Case

Associating Guardrails to agents via CDK seems to be a key feature of using agents in the first place. Without being able to automate this association we're manually calling the API above.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

all

### Environment details (OS name and version, etc.)

devcontainer

Start with the CfnAgent construct and compare its supported properties with the CloudFormation Bedrock Agent resource documentation and the Bedrock CreateAgent API's guardrailConfiguration parameter. Done means CDK exposes the guardrail association for agents and can automate it rather than requiring a manual API call; verify whether the upstream CloudFormation API supports it first.

Written by the indexing model from the issue text.