(apigateway): Can we please convert httpMethod of class ResourceBase to an Enum ?

### Describe the feature

Can we please convert `httpMethod` of class ResourceBase to an Enum ?
because it has only limited possible values in the AWS UI

```
const apigw = new LambdaRestApi(this, 'productApi', {
restApiName: 'Product Service',
handler: productFunction,
proxy: false,
})

const product = apigw.root.addResource('product');
product.addMethod('GET'); // product.addMethod(HttpMethods.GET) instead
```

### Use Case

This will help reduce error in typescript.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

"aws-cdk": "2.20.0"

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

Windows 10

Start by tracing the `ResourceBase` `httpMethod` value through the API Gateway entry point used by `product.addMethod('GET')`, and compare it with the proposed `HttpMethods.GET` usage. Check how existing TypeScript types expose allowed HTTP methods and identify the related API Gateway tests. Done means the supported methods are represented consistently without losing the current API behavior, with any breaking-change implications addressed.

Written by the indexing model from the issue text.