(aws-appsync): Introduce `addBedrockDataSource()` to `GraphqlApi`

### Describe the feature

Last re:Invent, [AppSync added data source integration for Bedrock](https://aws.amazon.com/about-aws/whats-new/2024/11/aws-appsync-ai-gateway-bedrock-integration-appsync-graphql/), but this functionality has not yet been added to the `GraphqlApi` construct.

### Use Case

- Create AI/ML powered GraphQL APIs
- Simplify integration between GraphQL and generative AI features
- Enable streamlined development of AI-enhanced applications

### Proposed Solution

Add a new method to the `GraphqlApi` construct:

```typescript
addBedrockDataSource(id: string, props: BedrockDataSourceProps): BedrockDataSource
```

This would allow developers to easily connect their AppSync GraphQL APIs with Amazon Bedrock models.

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.190.0

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

Ubuntu 24.04

The issue names the GraphqlApi construct but no source files or tests. Start at GraphqlApi and inspect how existing data-source methods are exposed, then trace the AppSync and Bedrock resource types needed for the proposed signature. Done means GraphqlApi provides addBedrockDataSource(id, props) and the construct behavior is covered by appropriate tests.

Written by the indexing model from the issue text.